Commutation in DC - DC choppers
Fig: A voltage commutated DC-DC Chopper and most significant waveforms
- In a voltage commutated thyristor circuit a voltage source is impressed across the SCR to be turned off, mostly by an auxiliary SCR. This voltage is comparable in magnitude to the operating voltages.
- The current in the conducting SCR is immediately quenched, however the reverse-biasing voltage must be maintained for a period greater than that required for the device to turn-off.
- With a large reverse voltage turning it off, the device offers the fastest turn-off time obtainable from that particular device. It is an exposition of ‘hard’ turn-off where the reverse biasing stress is maximum.
- ThM is the main SCR and ThAux is the Auxiliary.
- As a consequence of the previous cycle, Capacitor C is charged with the dot as positive.
- When the Main SCR is triggered, it carries the load current, which is held practically level by the large filter inductance, LF and the Free-wheeling diode.
- Additionally, the charged Capacitor swings half a cycle through ThM, L and D ending with a negative at the dot. The reverse voltage may be less than its positive value as some energy is lost in the various components in the path. The half cycle capacitor current adds to the load current and is taken by the Main SCR.
- With the negative at the dot C-ThAux is enabled to commutate ThM. When ThAux is triggered the negative charge of the capacitor is impressed onto ThM and it immediately turns off.
- The SCR does take the reverse recovery current in the process. Thereafter, the level load current charges the capacitor linearly to the supply voltage with the dot again as positive.
- The Load voltage peaks by the addition of the capacitor voltage to the supply when ThAux is triggered.
- The voltage falls as the capacitor discharges both changes being linear because of the level load current. When the Capacitor voltage returns to zero, the load voltage equals supply voltage. The turn-off time offered by the commutation circuit to the SCR lasts till this stage starting from the triggering of ThAux.
- Now the capacitor is progressively positively charged and the load voltage is equally diminished from the supply voltage. ThAux is naturally commutated when the capacitor is fully charged and a small excess voltage switches on the free wheeling diode. With the positive at the dot the capacitor is again ready for the next cycle. Here ThAux must be switched before ThM to charge C to desired polarity.
- Voltage commutation may be chosen for comparatively fast switching and it can be identified from the steep fall of the SCR current. There is no overlapping operation between the incoming and the outgoing devices and both currents fall and rise sharply. Stresses on all the three semiconductors can be expected to be high here.