Self oscillating PWM modulators, a topological comparision

High precision control of the output voltage or current of a switch mode converter with fast response is required for a number of applications. Dependent on the type of application, the desired precision and transient response can be difficult, if not impossible, to achieve with standard pulse width modulation (PWM) control caused by limitations in dynamic capabilities which often limits fast tracking of a reference signal, or fast settling during load steps due to too small achievable control loop bandwidth.

Achievable open loop bandwidth for standard voltage and current mode PWM modulators is typical in the f/sub s//10 or f/sub s///spl pi/ range respectively, where f/sub s/ is the switching frequency of the converter. For some applications this will require unacceptable high switching frequency to achieve enough control loop bandwidth for the desired dynamic performance.

With self oscillating modulators, the open loop bandwidth is equal to f/sub s/ which makes this type of modulators an excellent choice for a wide range of applications. Self oscillating PWM modulators can be made in a number of ways, either as voltage or current mode modulators, and the self oscillating behavior can be achieved either by using hysteresis control or by shaping the open loop function of the modulator so its gain and phase response causes a closed loop natural oscillation.

The two main types of self oscillating modulators have many similarities, but differences in dynamic performance and linearity are present. The work presented is related to the author's work with switch mode audio power amplifiers, where linear tracking of the reference signal is of major importance.

Use of the modulator topologies presented are not limited to this kind of equipment, but can be used in a very wide range of applications from very low to very high power levels.