There are various types of power supplies on the market, and the use of various resistors in these power supply designs greatly expands the range of choices. For clarity, the power supply referred to in this article is a device that has a fixed DC output of up to several thousand volts.

Regardless of the application, power supply designers must understand the specific safety or environmental regulations applicable to their field, as well as the actual electrical performance. This article will focus on how to use resistors to regulate the output of the power supply and protect it from failure.

The classification of power supplies usually depends on whether the input is AC or DC, and what type of regulation method is used to provide the correct DC output, usually switch mode or linear mode.

Power frequency line voltage is usually supplied by AC-DC power supplies, while batteries or any other DC power source provide DC-DC power. These DC-DC converters use switch mode technology to adjust the input voltage to a higher (boost) or lower (buck) output voltage.

Ready-made power supplies are suitable for many markets and general purposes, but custom design is required in some cases

Linear regulator

To understand the role of components in a power supply, it is necessary to understand the basic principles of how a power supply works. Many engineers remember designing a circuit like the one shown in Figure 1. This circuit uses a zener diode to provide a constant voltage for the load (R2). R1 is used to provide a minimum current to keep the zener diode in a constant breakdown state and to provide the load current.

Simple zener diode voltage regulator circuit

This type of system is suitable for circuits with low power and relatively stable supply voltage and load. If the load current drops or the supply voltage suddenly increases, it may exceed the rated power dissipation of the zener diode. The resistor in this circuit is easy to select, as long as its rated power meets the combined power requirements of the zener diode and the load.

For power supplies where the supply voltage or load may vary, a series design can use a pass transistor, which will ensure a stable load current and reduce the voltage output to the desired range.

Figure 2 shows this circuit. These designs usually use ICs or low dropout (LDO) regulators to regulate the load power supply. The divider formed by R1 and R2 senses and sets the voltage output relative to the reference voltage. If the circuit has a fixed output, the divider is located internally; for other applications, one or two resistors can be placed externally.

The resistor values are selected to provide the required ratio, and the most important consideration is accuracy. If the comparator circuit has high gain and high input impedance, it is easy to calculate the worst-case values using the formula in Figure 1, first choosing R1 maximum value and R2 minimum value, then choosing R2 maximum value and R1 minimum value. These calculations can show the maximum voltage deviation from the expected output.

Switching power supply

Since both the series pass device and the load consume energy, linear power supplies may have low efficiency. The efficiency will be lower as the voltage drop on the load increases.

Simplified diagram of a linear series regulator

To improve efficiency, designers often use another type of power supply topology. Switching power supplies (SMPS) use unregulated input DC voltage and switch it at a high frequency (10kHz to 1MHz). The duty cycle determines the DC output voltage after rectification and smoothing.

The regulation of SMPS output also uses a divider, but it regulates the switching frequency and duty cycle. By avoiding the losses caused by the linear regulator drop, SMPS can achieve up to 95% efficiency. Since high-frequency transformers and filter/storage capacitors are much smaller in size, SMPS may also be more compact than linear AC-DC power supply designs of similar power.

The main drawback of SMPS is that it requires a minimum load, and no-load conditions may damage the power supply. To avoid this situation, designers often use a power resistor as a dummy load. If the main load is disconnected, this resistor can be used to absorb a specific minimum load current. Of course, the dummy load resistor also has power dissipation, which affects the overall power supply efficiency, so this factor needs to be considered when selecting the resistor. Another way to avoid this problem is to use a shunt resistor at the output end when the load is open. For safety purposes, SMPS designs also use other resistors. Low-resistance, high-power resistors can prevent overvoltage situations. And current limiting designs can prevent short circuits.

This switching technology can also be used for DC-DC converter designs, which adjust one value of DC voltage to another. Buck converters are very similar in principle to the aforementioned SMPS design. Boost converters use charge pump technology to output a higher voltage than the input end. Both techniques use similar methods to regulate output voltage and provide circuit protection.

Other uses of resistors in power supply design

Discharge resistors are mainly used to discharge capacitors in circuits. They are connected in parallel with the load and are used to discharge smoothing capacitors and storage capacitors in AC-DC and DC-DC converters respectively. The capacitors remain charged after the power is turned off, which may cause harm to the user, so they need to be discharged. When choosing resistors for this task, two points need to be balanced: they should have high enough resistance values so that they consume little power when the circuit is working; and their resistance values should be low enough to discharge the capacitors quickly.

Surge limiting resistors can limit the surge current that may be caused by AC-DC power supplies when they are first turned on and the storage capacitors are charged. These resistors usually have low resistance values and are connected in series with the AC power line. For higher power supplies, negative temperature coefficient (NTC) resistors are usually used for this purpose. These resistors have resistance values that decrease with their own heating. One disadvantage of using such resistors is that the temperature must be kept constant during operation to ensure that they remain at low resistance values. A third solution is to use pulse-resistant resistors, which usually have power ratings in joules. It can better describe their function than normal continuous power ratings in watts.

Balancing resistors are used to modulate load currents when using multiple power supplies. Usually, using multiple DC-DC converters in parallel settings can be cheaper, more energy-efficient, and more compact than using a single high-power large power supply. When designing such circuits, it is not possible to simply connect the outputs together, and a method must be used to ensure that the load is shared evenly. Figure 3 shows that RSHARE resistor levels out the differences between converter outputs.

Balancing resistors share the load between DC-DC converters

This load sharing method is also used for other types of power supply designs, especially those that use power transistors. Multiple transistors are connected in parallel to supply power to the load, and load distribution resistors are used in series.

Another situation that requires balancing is shown in Figure 4. In this case, the storage capacitor is connected in series with the DC power output. The leakage current of the electrolytic capacitor acts like a resistor in parallel with the capacitor, as shown by RL1 and RL2 in the figure. These resistance values may vary considerably, and because they act as dividers across the output, they may cause a voltage difference across the capacitor that exceeds the capacitor's rated value. Matched resistors RB1 and RB2 cancel out this effect.

Balancing resistors ensure equal voltage across output capacitors

High voltage divider resistors are used to provide feedback to the control circuit in power supplies. These resistors are also used for other secondary purposes such as monitoring high voltage power supplies in defibrillators and charging energy storage capacitors and switching off the power supply at the expected charging level. High current detection is used to measure the supply current. This measurement method uses the principle of shunt current metering and requires a low value resistor to be connected in series and its voltage drop measured to calculate the current magnitude. The design of such circuits must take into account the selection of resistors, which require low resistance values to minimize heating and power consumption while also requiring high impedance for ease of measurement.

In summary, almost all resistor selections in power supply designs have different priority characteristics and performance requirements, including resistors that can handle high voltage, large current, and high power, as well as resistors with low tolerance. Resistors often need to have specific properties such as surge suppression or negative TCR.