DC DC Power Supply

Chapter 1: Principle of DC DC Power Supplies

This chapter will discuss what DC DC power supplies are, their construction, and how they function.

What is a DC DC Power Supply?

A DC DC power supply (also known as DC DC Converter) is a kind of DC power supply that uses DC voltage as input instead of AC/DC power supplies that rely on AC mains supply voltage as an input. The main purpose of DC DC power supplies is to produce a regulated voltage output for electronic and electric devices. Unlike AC voltage, DC voltage can’t be stepped up or stepped down using a transformer.

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).

Design of a DC DC Power Supply

This section explains the basic external components of a DC DC power supply and how to choose them. The components must be carefully chosen because they greatly affect the DC DC power supply’s properties in the figure below:

DC DC Power Supply Coil

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.

DC DC Power Supply Diode

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.

DC DC Load Capacitance

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.)

DC DC Power Supply Input Capacitor

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.

DC DC Power Supply External Transistor

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.

DC DC Power 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.

DC DC Power Supply Bipolar Transistor

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.

Base Resistance and Speed-Up Capacitance Value of a Bipolar Transistor

This is detailed as:

Base Resistance (RB)

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.

Speed-Up Capacitor (CB)

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.

How a DC DC Power Supply Functions

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.

Considerations When Choosing a DC DC Power Supply

The considerations when choosing DC DC power supplies include:

System Power Requirements

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.

Isolated vs Non-Isolated Configurations

DC DC power supplies are found in either non-isolated or isolated configurations. Isolated converters are built with the output and input circuits isolated galvanically, there is no path for DC between the output and input. These power supplies are at times used to separate the output and input circuitry for either dangerous voltage isolation or electrical noise. Isolated converters are found with many output voltages from one converter.

Non-isolated converters feature DC connection between output and input via common ground between output and input circuits. This type of converter can be made smaller and less expensive than isolated converters. Some non-isolated converters produce a negative voltage output from a positive voltage input.

Regulated vs Unregulated Output Voltage

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.

Mounting and Package Choices

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.

EMC and EMI Concerns

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.

Safety Requirements

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.

Chapter 2: Types of DC DC Power Supplies

The types of DC DC power supplies include:

Non-Isolated DC DC Power Supply

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.

Buck Converter or Step Down

In general a non-isolated buck converter or step down the voltage output (VOUT) is dependent on the voltage input (VIN) and D, the power switch’s switching duty cycle. This is a device that steps down DC input voltage. It is the average of the DC voltage that is reduced thus reduction in power.

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 Converter or Step Up

An elementary boost DC DC converter utilizes the same amount of passive devices but is laid out to step up the voltage input so that its output is more than the input. When the chopper is ON, current flows through the short circuit created by the chopper. No current goes to the load. During this time, the inductor L stores energy. This is the energy that is used to boost the output voltage at the load. Hence, the larger the inductance the greater the boost.

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

A general buck boost DC DC converter enables the DC voltage input to be either stepped-down or stepped-up, relying on the duty cycle. The voltage output is given as: VOUT = -VIN *D/(1 – D).

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.

Ćuk Converter

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.

Charge Pump

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.

Isolated DC DC Power Supplies

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.

Flyback Converter

The flyback type functions like a buck-boost but utilizes a transformer for storing energy.

Forward Converter

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.

For these types, the switching MOSFET in the circuits is represented by an ideal switch and the control circuit is not included. These are asynchronous type converters. However, if a MOSFET substitutes the normal diode rectifier for rectifying, it is referred to as synchronous rectifying and the converter is called synchronous DC DC converter.

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Chapter 3: Applications and Benefits of a DC DC Power Supply

This chapter will discuss the applications and benefits of a DC DC power supply.

Applications of a DC DC Power Supply

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.

Benefits of a DC DC Power Supply

• It offers techniques for extending voltage from partially reduced cell voltage.
• It is available as a complete hybrid circuit unit and needs very few additional parts.
• DC DC choppers are used to regulate voltage.
• It is designed to make best utilization of the power yield for PV systems.
• Isolated DC DC converters have more power transformers depending on the state of the barrier.
• The isolated converter output is organized as negative or positive.

Drawbacks of a DC DC Power Supply

• Switching converters are susceptible to noise.
• They are expensive.
• Choppers are insufficient due to unstable current and voltage supply.
• Drawbacks of flyback: More equated monthly installment due to gap, increased ripple current, increased output/input capacitance, and higher losses.

Specifications of DC DC Power Supplies

The specifications of DC DC power supplies include:

Efficiency of DC DC Power Supplies

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.

DC DC Current Rating

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.

Temperature Rating of DC DC Power Supply

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.

Ripple Voltage

This rating is a determination of ripple voltage amount on the output. A buck converter’s ripple voltage rating must meet customer needs.

DC DC Regulation

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.

Transient Response

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.

Voltage Rating

DC DC power supplies have thresholds on how far down or how far up they are able to transform a voltage.

Size and Weight

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.

Conclusion

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