AC DC Power Supply
Power supplies are electrical circuits and devices that are designed to convert mains power or electricity from any electric source to specific values of voltage and current for the target device...
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This article will take an in-depth look at DC DC power supplies.
The article will look at topics such as:
This chapter will discuss what DC DC power supplies are, their construction, and how they function.
A DC DC power supply is a device that converts electrical energy from one voltage and current level to another. When two electrical systems are operating different voltage levels, a DC DC power converter makes it possible to convert voltage from one level to the other with a minor amount of power loss.
Instead, a DC DC converter is utilized for this purpose. Hence, these types of DC power supplies may be considered equivalents of transformers. Like transformers, DC DC power supplies change the input voltage to a differing impedance level. It must be noted that no power is produced inside the power supply because all the output energy comes from the input energy. In practical applications, power losses happen inside the power supply; the energy is dissipated by some devices in the circuit. Due to progression in devices and circuit methods, DC DC power supplies may have efficiency reaching 90%. For the old models, efficiency generally ranges from 80 to 85%. Voltage level ranges from really low (low capacity batteries) to really high (very high power transmission).
The development of integrated circuits with advanced features and functions has made it feasible to operate power supplies at higher switching frequencies. This allows designers to decrease the size of magnetic components like transformers and output filter inductors in switching power supplies. Higher switching frequencies rapidly replenish the output capacitor’s charge enabling designers of state of the art power supplies to use smaller and fewer output capacitors while attaining fast and accurate dynamic output performance. The commercialization of wide band gap power semiconductors, such as silicon carbide (SiC) and GaN transistors, enable higher switching frequencies, with lower switching losses, enabling power supply designers to achieve higher power densities than is possible with Silicon MOSFETs.
There are numerous types of DC-DC converters offered on the market, including buck, boost, non-inverting buck-boost and flyback converters.
These four topologies are the dominated ones. The choice of which topology to use is a tradeoff between size, performance, and cost depending on the application requirements.
|Choosing External Components By Properties|
|Coil||CL*||SD*||External TR.||External TR.
|For Heigher Current||Small||Small||Large||Small
|(Low ON Resistance)||Small||Large|
|For Higher Efficiency||Light Load||Large||Small||-||Small IR||-||Large||Small|
|Heavy Load||Large||Small||-||Small VF||(Low ON Resistance)||Small||Large|
|For Low Ripple Output Voltage||Large||-||Large||-||-||-||-|
|For Better Transient Resonse||Small||-||Large||-||-||-||-|
For choosing the inductance value, the chart can be referenced below. It lists down values based on the oscillating frequency and current output or load. Oscillating frequency and inductance value are proportionally related hence this affects the choice of inductance. It’s better to utilize a coil with low DC current resistance whenever possible. When the inductance value decreases, the coil’s max current (IPEAK) rises, and the total current output becomes max with a particular inductance value.
When the inductance value rises, the switching transistor loss by max current reduces and efficiency increases with a particular inductance value. Also, the more the inductance value increases, the losses resulting from DC current resistance of the coil grows causing a reduction of efficiency. For coil selection, the rated permissible value of current is one of the essential parameters. If current gets over the rated permissible value of current, an inductance coil will dissipate heat. This results in coil magnetic saturation creating a decrease in efficiency. This will cause severe harm to the IC if the max current surpasses the permissible current value.
|Choosing Inductance Value|
A diode with low forward voltage is suggested as it avoids loss resulting from the forward voltage drop and increases efficiency. In a step-up circuitry, working start voltage also reduces. The forward voltage must be lower than 0.6V at the coil maximum current value.
The diode terminal’s capacitance should be low. If the capacitance is high then the switching speed is reduced and spike noise is increased out of the diode’s turn-off and on time. When the speed of switching is reduced, the switching loss is increased.
A diode that has a low reverse leak current must be chosen. With more reverse leak current, spike noise increases and efficiency reduces with the light load. Reverse leak current has a tendency to increase at high temperatures. Reverse leak current has a tendency also to increase with high current diodes (low VF).
For step up DC DC power supplies, the diode rated current must be three or two times more than the maximum coil current at the voltage input minimum value to be utilized. For step-down DC DC power supplies, the rated current must be three or two times more than the maximum coil current at the voltage input maximum value to be utilized. With PFM control, it must be noted that the maximum current value will reduce so caution must be taken.
The diode rated voltage must be one and a half times more than the voltage output for step up DC DC power supplies and one and a half times more than the voltage input for step down DC DC power supplies. In practice, the terminal's voltage must not be surpassed by the rated voltage value.
When utilizing a ceramic load capacitor, the temperature characteristics must be known. Capacitors besides those with B properties might not function normally due to a decrease in electric capacity conferring to ambient temperatures or DC bias properties. The operation of tantalum capacitors, aluminum electrolytic capacitors, and OS-CONs must be checked when they are used.
When utilizing tantalum capacitors, a capacitor with at about 10mF capacitors must be used. For applications that need a current output of higher than 100 milliamps, a tantalum capacitor with a higher than 100mF load capacitance must be connected. The equivalent serial resistance for the load capacitor must be close to 0.1W to 0.5W. If a low ESR capacitor like an OS-CON is utilized as a load capacitor, an atypical operation might occur because of insufficient phase compensation. Therefore, ceramic capacitors must not be used. Even if OS-CON, tantalum, or aluminum electrolytic capacitors are utilized with IC’s hacaparmonious with capacitors, the functioning must be checked fully.
When using an aluminum electrolytic capacitor, the load capacitance value must be three or two times more than the value suggested in the general application circuitry so as to prevent any decrease of capacitance in low temperature and upsurges in ESR. In such instances, connect a tantalum capacitor, 10mF or above, or a ceramic capacitor of 0.1mF – 1.0mF must be connected in series so as to decrease ESR. The permissible ripple current is the significant parameter for the aluminum electrolytic capacitor. Excessive ripple voltage produces heat and reduces the IC’s life. (Lower than 50mV ripple voltage output must be selected.)
The input capacitor acts as the power supply ripple elimination capacitor for the IC for step down DC DC power supplies. The capacitor must be closely connected to the IC.
The input capacitor and the step up DC DC power supply IC must be connected because the capacitor lowers the input power supply’s impedance influence on the IC. Unlike load capacitors, small ESR values may be chosen irrespective of the capacitor types.
When the voltage input is lower than 1.2V, there might not be enough gate voltage to switch on a Power MOSFET. In this instance, a bipolar transistor must be used. For more output current, a Power MOSFET with small ON resistance must be used. The bipolar transistor typically has a less (hFE) current amplification rate. Hence, when a bipolar transistor is utilized for high currents, the base current increases and efficiency reduces in comparison to a MOSFET.
A power MOSFET with output capacitance and input capacitance lower than 1000pF must be used. A MOSFET with a high switching speed must be selected. Efficiency rises when the switching speed goes up. [High switching speed: small switch on time delay, rise time, switch off delay time]
A MOSFET with a source to gate cut off voltage much less than the voltage input must be selected. A bipolar transistor is appropriate when the voltage supply is less than 1.2V. When powering the ICs for step up DC DC power supply, a voltage higher than the source to gate cut off voltage must be input to the power supply pin.
ON resistance between source and drain of the Power MOSFET must be low. However, Power MOSFETs with low ON resistance generally have big output and input capacitance. There’s a trade-off among output capacitance, input capacitance, and ON resistance.
For the step up DC DC power supplies, a MOSFET must be chosen with a rated current three or two times higher than peak current. For step down DC DC power supplies, a rated current close to two times that of the current output multiplied by step down ratio must be selected. A practical device must check the current waveform and the MOSFET heat before choosing. It is also important to observe the current peak value closely since the current peak increases mostly in PFM control.
The rated voltage for the MOSFET for step up DC-DC power supply ICs must be one and a half times bigger than the voltage output. The rated voltage for step down DC-DC power supply ICs must be one and a half times bigger than the voltage input. Actual parts must be checked before using and prevent a case where the voltage between the pins surpasses the rated voltage.
If it happens that every loss of the circuitry while efficiency reduces is taken up by the transistor, the MOSFET power dissipation must be bigger than the losses. When the voltage output and the current output are high, the MOSFET must have enough margin to resist the power loss. The heat falls in the functioning temperature range must be checked, and if important, the heat dissipation must be considered.
Current gain rate (hFE) must be chosen from the 100 to 500 range. Bipolar transistors with a very high current gain rate generally have a low base current and more OFF leak current.
Bipolar MOSFETs with high switching speeds must be utilized as much as possible. Efficiency increases when switching speed goes up. Also a MOSFET with an output collector capacitance of about 10pF must be used. This includes high switching speed, small turn on time, fall time, and storage time.
This is detailed as:
Base resistance must be chosen within 250Ω to 2kΩ range because resistance lower than 250Ω affects the IC's operation. With a small base resistance value of about 200Ω to 500Ω, the current output goes up but efficiency decreases at light loads. With a high base resistance value of close to 700Ω ~ 2kΩ, the current output goes down but efficiency goes up at the light load.
The speed-up capacitor (CB) is used to enhance efficiency. The speed up capacitor value is unchangeable by the base resistance value and switching regulator oscillation. If the speed up capacitor value is enlarged greatly, switching speed increases and current supply increases. Even if the speed up capacitor value goes up marginally, resulting changes in the switching speed will occur.
DC DC power supplies generate a constant and precise current proving to be an important aspect in mission critical electronic parts development. A DC DC power supply receives DC voltage and then converts it to an appropriate DC voltage level using a circuit of inductors, diodes and power transistors. Once the voltage is converted it is then smoothened by specific capacitors and regulated at the output. They are progressively being utilized in portable electronic gadgets like mobile phones and laptop computers. Previously and before the manufacture of power semiconductors, the major method to convert the voltage of a DC power supply to a higher voltage was by using AC.
The considerations when choosing DC DC power supplies include:
As with many power supplies, a primary consideration when selecting a DC DC power supply is to find the load output power needs of the system. The output needs of the DC DC power supply include the input current and output voltage. The output voltage might have tolerance ratings depending on environmental conditions like output load current, input voltage, ambient temperature, etc. The load current need specifications must include the maximum, minimum, and typical values.
Required specifications for DC DC power supply applications are distinctive compared to AC/DC power supplies as the input voltage is not standard, since they are for AC/DC power supplies. When choosing a DC DC converter, it is important to stipulate the input voltage range that will be used on the power supply.
DC DC power supplies are found in either non-isolated or isolated configurations. When a transformer is incorporated into the powertrain of converters, it provides isolation between the primary and secondary side and forms an isolated converter. Buck converters with a transformer are called forward converters since the energy transfer happens when the primary side is active or conducting.
A boost converter with a transformer is called a flyback converter since the energy is stored in the magnetics during the primary switch ON time and transferred to secondary or output side when the Primary Switch is OFF. Several power supply companies offer these types of isolated power converters, which are known as power bricks in the industry.
Any DC DC power supplies generate a strongly regulated voltage output, just like AC/DC power supplies. Small or cheap power supplies may be utilized in applications that can handle unregulated voltage output. The choice of unregulated or regulated voltage output is at times available on only low power converters.
DC DC power supplies are found in a variety of mounting styles and packages. In applications where the power is mounted directly on a PCB there are options of surface mounting) or through hole mounting and single in-line pin or dual in-line pin setups. Chassis mounted power supplies are available for uses needing that mounting style. Most power supplies are found in DIN rail mounting setups for industrial applications. Both encapsulated and open frame power supplies may be found in many mounting and package configurations.
Most electronic equipment for sale is required to meet EMC and EMI (Electromagnetic Compatibility and Electromagnetic Interference) regulation requirements. The regulation requirements aim to ascertain that the equipment does not affect the working of other equipment and that external electrical noise will not prevent the certified equipment from working properly. DC DC power supplies may be certified and be in compliance with regulatory requirements. However, in many applications the complete system is certified. Small circuits on the inside do not need certification.
Similar to EMC and EMI regulation requirements, much electronic equipment sold is needed to meet safety regulation requirements. As with the EMC and EMI regulation certification, equipment are given safety certifications for the end equipment. Certifications are at times not needed for the internal sub-parts but may be obtained if necessary. Safety certification for a power supply must also be attained if the DC DC supplies are utilized to isolate equipment operators from dangerous voltages.
A power supply consists of a switching system that breaks a constant source of power into controllable increments of energy. The switching system is followed by a filtering system to reconstitute the increments back into a steady source of usable output power.
In its simplest form, a non-isolated power supply consists of four basic components - a switching transistor, a diode, an inductor, and an output capacitor. Multiple topologies can be implemented using these four circuit elements and the appropriate control circuitry to achieve the desired performance objectives for output voltage and efficiency.
In many applications, power is distributed at higher voltage levels to achieve optimal or required system efficiency levels. In those power applications, the task of the power converter is to step down the incoming voltage to a lower value required by the load. These types of power converters that have an output voltage lower than the input voltage are called Buck Converters.
In essence, a switched DC DC power supply or a regulator is circuitry which utilizes an inductor, a power switch, a capacitor, and a diode to transfer the power from the input to the output. These can be categorized in many ways to realize the following types.
When the switch (SW) is closed or ON, the energy is transmitted from V in to the load. By controlling the ON and OFF times of the switch, the energy transferred or transmitted to the output can be varied. The ratio of the switch ON time to the period, switch ON time + switch OFF time, is called the duty cycle of a power converter, a number between 0 and 1. The output voltage of a power converter can be changed from 0 V to almost V in by controlling the SW ON time.
In the case of the buck converter, the inductor is connected directly to the output capacitor and load. Inductor current can flow continuously through the diode to the output even when the switch is open through the antiparallel diode. When used as a regulated output converter, the duty cycle is regulated or controlled by the power supply controller to maintain the desired constant output voltage regardless of the changes in V in or system parameters. Non-isolated buck converters can achieve very high efficiencies.
When the chopper is switched ON, the source and the load are directly connected to each other and the output VO equals the input voltage Vs. ON time is the period which the chopper is turned on for. It is represented by TON.
For the period of activation of the chopper, current builds inside the load at an exponential rate and reaches its peak value when TON reaches its end. We assume the TON is short enough for the load current not to reach steady state. As long as the TON is short enough, the increase in current may be approximated into a line.
Boost Converters use the same circuit elements, which are arranged in a different format to provide different performance characteristics. Boost converters increase the input voltage to provide an output voltage greater than the input voltage. In a boost converter, the power is transmitted to the output in a two-step process with the inductor acting as a temporary or interim storage element. When the switch is closed, the current flows into the inductor the longer the switch is ON, which allows for higher magnetic energy to be stored.
The diode blocks current from flowing back from the load as the output voltage is held up by the output capacitor. When the switch is opened, the energy stored in the magnetic field of the inductor flows through the diode, recharging the capacitor and replenishing the energy delivered by the capacitor to the load when the switch SW was ON. This two-step process results in high peak currents in the components and impacts the efficiency of higher power applications. This topology is limited to low power applications.
As CH switches OFF, the current through L does not collapse instantly but rather slowly decays. As a result, the current is pushed through D and the load during TOFF. The emf that’s induced in L is reversed. Because of this, the load voltage equals the sum of inductor emf and source voltage. VO = VS + L(di/dt). Thus, the voltage input is stepped up.
Buck boost converters are a hybrid integration of buck and boost power converters. They combine the buck circuit on the input and the boost on the output side with a shared inductor between them. The advantage of this topology is its ability to regulate the output voltage whether the input voltage is above or below the desired output voltage. The output remains steady as the battery moves from a fully charged state to a fully discharged state.
These types of power converters are popular in battery powered applications where the output load voltage must be maintained even when the battery is discharged to its lowest operating limit. The benefit of this approach for battery powered end products is an extension of run times and the maintenance of performance as the battery discharges.
When V in is higher than V out, (about 14.5V when a fully charged battery is connected in a 12V load system), the circuit operates in buck mode with the boost switch remaining open and the output diode continuously conducting. When the input falls below the set V out level (while the battery is being discharged), the input switch will remain ON, and the circuit will behave like a boost converter to maintain the output voltage. A single control circuit performs the output voltage control and determines whether the buck or boost operating mode is required.
As seen from the above equation, the voltage output is reversed in polarity always with respect to the input. Therefore, a buck boost circuit is also called a voltage inverter.
Cuk converters may be seen as an integration of buck converter and boost converter, with one switching component and a common capacitor, for coupling the energy.
Just like the buck boost circuit with inverting topology, voltage output of non-isolated Cuk converters is generally inverted, with higher or lower values depending on the input voltage. Typically in DC DC converters, the inductor is utilized as a primary energy storing component. In a cuk converter, the primary energy storing part is the capacitor.
A charge pump is a type of DC DC converter that utilizes capacitors to store charge to increase or reduce voltage. Charge-pump circuits are able to operate at high efficiency, at times reaching 90 to 95%, yet being simple circuits.
There are two primary configurations of transformer based isolated DC DC power supplies: flyback and forward. In both these configurations, the transformer offers the isolation between the output and the input.
The flyback type functions like a buck-boost but utilizes a transformer for storing energy.
In the forward converter, the transformer is utilized in a normal way to transfer power from the primary side to the secondary side when the switch is closed.
Synchronous Rectifiers used in Power Supplies: In DC DC converters, diodes perform output rectification in switching circuits. When using regular power diodes, such as freewheeling diodes or rectifying diodes, the conduction losses can be unbearably high, especially at high currents where VF is close to 0.6 to 0.7 volts.
To lower losses, power MOSFETs are used in place of rectifier diodes and freewheeling diodes. The power losses are significantly improved compared to standard diodes or Schottky diodes. Driving the power MOSFETs synchronously with the main power switches can be a complex step requiring special controllers. The benefits, however, outweigh the complexity and cost, which is an approach that has been broadly adopted in all types of switching power supplies.
DC DC power supplies are used in many products since most electronic devices require DC power, such as portable electronic gadgets like laptop computers and mobile phones, which are sourced with energy from batteries. Electronic gadgets have several small circuits with each circuit having its own requirements and level of voltage that differs from the one supplied by the battery. Larger DC DC power supplies are used to optimize the power yield from photovoltaic systems and to charge batteries. They regulate the voltage output with some being able to regulate the current for LED power supplies.
DC DC power supplies are applied in portable electronic gadgets such as laptop computers and mobile phones, which are sourced with energy from batteries primarily. Such electronic gadgets often house several small circuits, each having its own requirement for the level of voltage differing from the one supplied by the battery or an exterior supply (often lower or higher than that of the supply voltage).
Additionally, the battery voltage drops when its stored energy is depleted. Switched DC DC power supplies offer a way to up voltage from a partly lowered battery voltage thus saving space. Utilizing many batteries to attain the same thing would take up space.
Most DC DC power supplies also regulate the voltage output. Some exceptions involve high efficiency LED power supplies, which are a kind of DC DC converter that regulates current through LEDs, and uncomplicated charge pumps that triple or double the output voltage.
DC DC power supplies are designed to exploit harvesting energy through wind turbines and for PV systems are known as power optimizers. Transformers utilized for converting voltage at mains supply frequencies of 50 to 60 Hz should be heavy and bigger for powers beyond a few watts.
This makes these power supplies costly, as they are prone to energy loss in the windings and eddy currents in the cores. DC DC methods which utilize inductors or transformers operate at high frequency, needing only very small, light, and cheap wound components. These methods are utilized even when a main transformer would be utilized. For instance, for domestic electronic equipment it is desirable to rectify mains supply voltage to DC, using switch-mode methods to change it to high frequency AC at the needed voltage, then rectify to DC voltage.
The entire complicated circuitry is cost effective and very efficient compared to a basic mains transformer circuitry of the same output. DC DC power supplies are broadly utilized for DC micro grid applications, in the setting of other voltage levels.
The specifications of DC DC power supplies include:
Efficiency is a fraction of power input which reaches the load. Some DC DC power supplies are 90% efficient or more. When utilizing a DC DC power supply, it is good to ascertain that the supply offering energy to the DC DC power supply can offer enough energy to make up for the inefficiency. A good rule of thumb is assuming a DC DC power supply is efficient 80%, and then supply with 125% the load power used. For instance, if a load of 4W is required, a 4 W DC DC must be used, driven by a 5W supply. Efficiency for DC DC power supplies is generally stated in curves, with maximum efficiency obtained at a certain load current. Efficiency might be reduced at low output power, where the power needed to power the circuitry is relative to the load power.
This is the peak current amount that a user must try to have the DC DC power supply offer to a load. The DC DC power supply may offer above this current amount, but will generate heat, and might fail.
This is the peak ambient temperature which the DC DC power supply must operate under full load. When pushed past this safety threshold, the DC DC power supply might overheat and get damaged, or might power off as a protective measure.
This rating is a determination of ripple voltage amount on the output. A buck converter’s ripple voltage rating must meet customer needs.
This number of merits is associated with how tight the output voltage is controlled over load current and input voltage. If a DC DC power supply has a 1%regulation rating, then the voltage output will not vary higher than 1% from nominal value over the stated output current range and input voltage range.
DC DC power supplies utilize closed feedback loops to offer regulated outputs. Shifting in input voltage or load current may result in temporary shifts. Speed of control loop responses give an idea of the duration it takes for the power supply to respond to shifting conditions and obtain the voltage output in regulation.
DC DC power supplies have thresholds on how far down or how far up they are able to transform a voltage.
DC DC power supplies can work at really high frequency, allowing them to be miniaturized. Because other mechanism losses increase with frequency, resulting in less efficiency, there is somehow a trade-off between efficiency and size.
DC DC power supplies are found in a variety of mounting styles and packages. In applications where the power is mounted directly on a PCB, there are options for surface mounting, through hole mounting, and single in-line pin or dual in-line pin setups. Chassis mountings are also available. DIN rail mounting setups are ideal for industrial applications. Encapsulated and open frame power supplies have several mounting and package configurations.
Electronic equipment is required to meet Electromagnetic Compatibility (EMC) and Electromagnetic Interference (EMI) regulation requirements. These regulations are to ensure that electronic equipment will not affect the workings of other equipment and that external electrical noise will not prevent certified equipment from working properly. DC DC power supplies may be certified to be in compliance with regulatory requirements. Most applications have the complete system certified. Small inside circuits do not require certification.
Similar to EMC and EMI regulations, electronic equipment has to meet safety regulations. As with the EMC and EMI regulation certification, safety certifications are for the end equipment and not required for internal sub-parts, but can be obtained if necessary. Safety certification for a power supply is necessary if a DC DC supply is used to isolate equipment operators from dangerous voltage.
DC/DC power supplies otherwise known as DC/DC converters are power supplies that convert a DC voltage of a certain magnitude to one of a different magnitude to supply a device. They are quite important these days since most electronic devices require DC power for Industrial, Medical, and Telecom markets. They are classified into isolated and non-isolated converter topologies, which are chosen based on the needs of an application.
Isolated power supplies are driven by the Telecom industry, which uses a negative (-48V) bus system for long term reliability. Advanced wireless systems have become a major player in the communications industry. The quest for higher efficiency and power density has been driven by the adoption of smaller and higher performing non-isolated converters.
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