The scope of this video is to make current transformers for soft switching inverters, in either Tesla coil, induction heaters or other inverter types using resonant switching. This video end article, should enable you to identify the correct construction according to your inverter design limitations.
Key points appearing in the video. This post will not be a complete design article, it’s goal is for you to understand enough of the CT function to make a working one, not for you to win a Nobel prize in current transformer design.
Current Transformers and DRSSTC Tesla Coils
A current transformer is used for two different functions in a DRSSTC:
- Feedback from primary current, to ensure zero current switching
- Overcurrent detection, to protect IGBTs from excessive peak current
Feedback for zero current switching needs to have as little phase shift as possible, to have a signal that is as true as possible to the primary current.
Overcurrent detection signal is usually full-wave rectified and fed into a comparator or op-amp, so here a DC level signal is used for indication of the current level. Phase shift is not as important, but excessive phase shift should still be avoided.
Current Transformer Theory
Turns ratio transformer theory applies to both regular mains transformers and high frequency current transformers. In a step up transformer, the voltage is the ratio difference higher on the secondary side, than on the primary side. According to Ohm’s law, the current is opposite. This is the principle of the current transformer, so it is essentially a high voltage step up transformer.
Examples and comparison between iron core mains transformers and high frequency current transformers. Examples includes cascaded transformers to illustrate the above principle.
A current transformer with a shunt resistor of 0 Ohm, will have 0 phase shift. At this operating point, the primary current will be in phase with the secondary current. According to Ohm’s law, we do however need some resistance, to have a resulting voltage output to use in our driver circuit.
A current transformer with a shunt resistor of infinite Ohm, will have 90 degree phase shift and the primary current would be completely out of phase from the secondary current, making the signal useless for zero current switching feedback.
We need a resistor that is low enough to not introduce too much phase shift and one that is high enough to develop a useful voltage. We also need to take the power dissipation of the shunt resistor into account.
Construction of Current Transformers
Use a suitable ferrite material, which can be virtually the same size core and material as you use for the Gate Drive Transformer ( GDT ). A good starting point could be a 30-40 mm diameter ring core, 10 mm wide, N30 ferrite material (or similar properties from another material).
Ethernet single wires can be used for winding the coils. Same as can be used for the GDT.
Be sure to place the CT at either the IGBT output leg to the resonant capacitor / MMC or to the primary coil. Never place it between the resonant capacitor / MMC and the primary coil. The high current ringup in the primary LC circuit equals a high voltage potential difference to other circuits, high risk of flashover to the CT.
Never run a CT unloaded or without any kind of termination. Being a high voltage step up transformer, the output voltage of a unloaded current transformer is easily in the range 20-50 kV and that will flashover and destroy your CT.
Never use wire wound power resistors in high frequency circuits. Though it might be tempting to deal with the power dissipation of a low resistance shunt, there is huge disadvantages. A wire wound resistor is a coil and at high frequency it will introduce increased inductance and thus provide excessive phase shift.
Use metal film or carbon film resistors. Metal film resistors is sturdy, precise and comes in both high precision and wattage ratings.
Quick Design Table for Current Transformers
This table provides calculations for 3 different primary current situations. 500 A, 1000 A and 2000 A is listed in the 3 horizontal tables to the left. The tables on the right are power dissipation calculations.
I have marked the turn ratios to shunt resistors that are most suitable for the 3 operating points and the power dissipations that are within the limits of a good metal film resistor and with a derating according to the duty cycle of a DRSSTC around 5-10%. So the listed Watt values can at least be divided by 10, as they are listed for DC voltage.
