PWM in power supply design

Switching mode power supplies use semiconductor switches (usually MOSFETs) to drive magnetic components, usually transformers or inductors. The output of the switching power supply circuit is then rectified and adjusted to provide a DC output. Switch-mode power supplies are popular because they offer higher efficiency than non-switch-mode power supplies such as linear regulators.

What is pulse-width modulation

Pulse width modulation (PWM), also known as pulse duration modulation (PDM), is a technique that reduces the average power of alternating current (AC) signals. Effectively truncating part of the waveform reduces the average voltage without affecting the fundamental frequency of the signal. Increasing the time the voltage is "off" reduces the average voltage and thus the power.

Use PWM output control

A switch-mode power supply must implement a feedback control loop to keep its output voltage within the desired limits as the load changes -- the power supply's output voltage is fed back through an error amplifier to provide a control signal. The most common method of control is to use pulse-width modulation (PWM). Adjust the pulse width of the AC signal at the input end of the power supply to increase or decrease the power supply, which in turn translates into a change in the voltage at the output end of the power supply. For example, increasing the input pulse width increases the output voltage, and decreasing the pulse width decreases the output voltage. This mechanism provides closed-loop feedback control of the output voltage.

One problem to keep in mind is that typical AC waveforms tend to have benign rising and falling edges. When PWM control is applied, rising and falling edges become more abrupt, especially if the duty cycle is relatively small. Sudden voltage changes create transients that cause electromagnetic noise and large surge currents within the circuit. In addition, small errors in the control circuit may amplify into serious output errors, which may cause the output voltage to be unstable. The standard solution is to avoid sudden on-off switching of the input waveform and instead use slope compensation techniques to limit the rate of change.

Peak current mode control (PCMC) technology provides a simple solution for PWM power sources, except for inductor-to-inductor-to-capacitor (LLC) converters that require voltage mode control. As duty cycles approach maximum values, PWM control is always challenging. Designing circuits to avoid this is always preferable to adding additional control circuits to apply slope compensation to prevent output instability.

Design considerations

Transient starting current

One of the disadvantages of switch-mode power supplies, especially when used to isolate a power supply, is that powering up the inductive element of the power supply while on can cause considerable transient current. In addition, the initial current is unpredictable; When the inductor is first energized, it varies with the exact point in the AC cycle.

The PWM based control circuit can implement the soft start function, which controls the initial power-on phase to limit the available energy of the circuit and limit the excitation current until the power supply reaches the steady-state condition. Limiting the initial surge current protects the component and reduces noise emissions associated with transient currents.

Overcurrent protection

One benefit of PWM control is that current detection logic can be used to disable power by turning off PWM if the output current exceeds a defined limit. This provides an easily implemented overcurrent protection mechanism that automatically resets once the current is returned within its boundaries.

Manage low loads using pulse frequency modulation

One of the main drawbacks of PWM in switch-mode power supplies is their inherent inefficiency at very low loads. Under no-load conditions, the power supply will continue to lose due to the power control circuit. This can be a problem for battery-powered devices that operate for long periods of time in standby mode, where the efficiency of the power supply determines battery life.

The solution to this situation is to replace PWM with pulse-frequency modulation (PFM). Here, the duty cycle of the AC waveform is unchanged, and the control of the power output is achieved by changing the frequency of the AC input.

The main problem with PFMS is that noise filtering designs become more challenging because noise is generated over a wider frequency range.

The other problem is that PFM control produces much larger output voltage ripple than PWM control, and transient response times may be longer. These problems can make a designer's task more difficult if the power supply drives components that are sensitive to voltage fluctuations, especially integrated circuits.

The power chip now offers built-in dual-mode PWM and PFM controls that can be switched automatically based on output load. Thus, by definition, limiting PFM control to low load conditions will minimize the effects of adverse effects such as emission noise and voltage ripple.

Manage low loads using pulse jump modulation

Another technique for managing low load conditions is to turn off the PWM waveform for a short period of time and rely on the output capacitor of the power supply to maintain the output voltage during this time. This process of disabling the PWM waveform is called pulse jump or pulse jump modulation (PSM). Under no-load conditions, the PWM waveform only needs to be turned on intermittently for a short time to compensate for the loss of the power supply itself that consumes the output capacitor.

conclusion

The main advantages of using PWM are the use of very high frequencies to optimize circuit design due to high efficiency, very low power loss. It is also relatively inexpensive to implement compared to similar technologies for power supply design and can handle high loads. The main drawback is the additional complexity required to manage low loads. However, the availability of integrated devices that combine PWM control with automatic low load management simplifies this task for power designers.