Pulse Droop Compensation using a PWM Technique

Applications that require high energy long pulses are frequently produced by using capacitor banks switched to the load by controlled switches such as IGCTs or IGBTs. To ensure low droop the capacitor bank must store significantly more energy than that required for the pulse. For applications with higher pulse energy the capacitor stored energy to ensure adequate pulse flatness can become prohibitively large, expensive, and require elaborate protection systems. Systems to compensate for the droop have been developed that permit significant droop on the capacitor bank and compensate for this by electronic means. The result is that the total stored energy is only slightly greater than the required pulse energy. One of the most popular circuits used for droop compensation is known as the bouncer [1]. An alternative to the bouncer has been developed and this uses a switched mode Pulse Width Modulated (PWM) technique that in conjunction with the pulse transformer inevitable residuals of leakage inductance and shunt capacitance can provide a droop compensation method with low ripple. The PWM compensates for the droop while the pulse transformer residuals reduce the switching ripple to acceptable levels. An important characteristic of the system is the ability to compensate for voltage droop characteristics that are non-linear and it particularly adapts well to digitally implemented control systems. The paper will describe the underlying theory, and show worked examples and detail initial results from an application.