Trigger Transformer Theory
As the word trigger implies, a trigger transformer is used in a circuit that initiates some sort of action or event. Once initiated, some applications may no longer require continued presence of a voltage to complete the action or event. Other applications may need the voltage but for a limited amount of time. Regardless, the application provides a voltage pulse to the trigger transformer’s primary. The trigger transformer’s turns ratio steps up or steps down the secondary voltage as needed. The trigger transformer’s secondary then supplies voltage or current to its load. The load is usually the gate of a semiconductor switch such as a transistor, F.E.T., S.C.R., etc.. The trigger transformer also provides voltage isolation between the primary side circuit and the secondary side circuit. Most circuit designers would refer to the trigger transformer as a type of pulse transformer. Refer to Pulse Transformer Theory for additional information.
One example of a trigger transformer application is the electronic flash in a traditional camera. A basic circuit is shown in Figure 1.
A charging circuit takes energy from a battery and charges two electrolytic capacitors (approx. 300V). The negative sides are both connected to ground. One capacitor is much larger than the other. It is connected to the electrodes of a glass tube filled with xenon gas. This capacitor provides the energy needed to produce the flash, but lacks sufficient voltage to initiate the flash. The primary of the trigger transformer is attached to the positive side of the smaller capacitor through a switch. The trigger transformer secondary is connected to a metal plate(s) or grid(s) that partially surrounds the glass tube. The trigger transformer is designed to step up the voltage to high voltage levels. When the switch is closed the trigger transformer places high voltage across the plates. The high voltage ionizes the gas inside the tube. The gas becomes conductive. The large capacitor discharges through the gas thereby producing a bright white flash. The capacitor rapidly discharges its energy and must be recharged to produce another flash. The switch between the trigger transformer and the smaller capacitor is opened. A small drain resistor is placed across the high voltage plates to discharge the voltage on the plates. In this example the trigger transformer aided the initiation (or triggering) of the flash by delivering a stepped up voltage pulse. Figure 1 shows the trigger transformer windings grounded together. With proper circuit design the trigger transformer could also provide voltage isolation.
In the preceding example, the trigger transformer (which is a pulse electronic transformer) design does not saturate the core and usually employs unipolar core utilization. There are trigger transformer applications that use bipolar core utilization and/or intentionally saturates the core. Bipolar core utilization indicates that the magnetic flux alternates between positive and negative directions. Unipolar core utilization indicates that the flux direction remains either positive or negative. Examples are found in the Royer Inverter Circuit and the closely related Jensen Circuit.These are shown in Figure 2A and Figure 2B.
Simply stated, transformer saturation repeatedly occurs in alternating directions which in turn triggers (switches) the transistors on and off in alternating fashion, thereby creating an A.C. output voltage. The switching of the transistors forces the current direction to alternate which then forces the alternating direction of core saturation.
Figure 3 below is a unipolar application which shows how a trigger transformer can use core saturation to shorten the time duration of a pulse. The trigger transformer usually has a high impedance load (lightly loaded) hence it acts much like a saturated inductor but with voltage step up or step down capability and voltage isolation. The primary winding of the trigger transformer has much higher impedance than the series resistor until saturation occurs. Before saturation, most of the circuit’s voltage drop is across the trigger transformer’s primary. The trigger transformer’s turns ratio can adjust the secondary output voltage. There will be voltage droop. After saturation, most of the voltage drop is across the resistor, the secondary output voltage is substantially reduced, and the time duration of the output pulse has been reduced. The pulse’s time duration can be calculated from the transformer’s volt-second product.
Gowanda designs and manufactures pulse transformers and trigger transformers in a wide variety of shapes and sizes. This includes various standard types of core with bobbin structures (E, EP, EFD, PQ, POT, U and others), toroids and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. Our capabilities include foil windings, litz wire windings and perfect layering. For toroids, the list includes sector winding, progressive winding, bank winding and progressive bank winding. Gowanda has a variety of winding machines, including programmable automated machines and a taping machine for toroids. Gowanda has vacuum chambers for vacuum impregnation and can also encapsulate. To ensure quality, Gowanda utilizes programmable automated testing machines. Most of our production is 100% tested on these machines.