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ADuM7234 H-Bridge for induction heating

Question asked by jorgemoura on Jun 25, 2017
Latest reply on Jul 11, 2017 by RSchnell


I'm building an H-Bridge power stage switch to drive and RLC circuit which features an induction coil and capacitor tank coupled by an 10:1 toroidal transformer to the H-bridge output, working in the frequency interval of [60,90] KHz. To maintain the RLC locked on its resonance frequency I used an PPL that reads the frequency of the coil and locks on it.

I'm trying to use the ADuM7234 to drive the MOSFETS or IGBTs at 80V maximum and maximum current that can reach 120A, limited by the power source. I don't believe that so much current will be needed because I already built an H-Bridge based on the Linear LT1162 IC that when working drained 20A at 40V.

I intend to use this induction heater to heat 25um thick, 1cm2 copper foils until 1050ºC as fast as I can inside a reactor with controlled atmosphere.


So the problems and questions:


- The output of the PLL (CD4046) and the input in the BNC plug are a 12Vpp square wave, so I put the TLP2955 and TLP2958 opto-couplers to reference the signal to 5V and invert it also. Do you have other component suggestion that make the same result but with less delay and low noise sensitivity?


- The output of the opto-couplers (Vopp) decreases when I plug them to the ViA and ViB and I believe that it is an impedance mismatch since when I remove the 1K resistance (R3 and R4) the signal gets better. So, what is the input impedance of the ADuM7234 and does it varies when on and off?


- I'm still testing the assembly in the breadboard so I understand that elevated parasitic impedances can be present but the chips overheat too fast, can this be from a problem in the circuit topology?


- I'm using a computer power source to get the 5V and 12V that have a common ground, should I use two separated power sources to have isolated grounds? Should I use separated power sources, one for each ADuM7234?


- The Disable pin needs to be grounded for the drivers to work?


- The gate resistors are 3.3Ohm and not 1K, can I put just a ferrite instead of the gate resistor to reduce the initial turn on ripple?


- The LT1162 IC used a resistor between the upper MOSFETs gate and source, but from the Analog application note ( Inside iCoupler® Technology: Driving an H Bridge with ADuM3220 Isolated Gate Drivers) it's not included, is there a reason for this, or should I include one close the loop during in the charging cycle of the bootstrap capacitors?


- When I increase the power (VPP), the gate signal gets distorted, what can be the reason for this?


- Is the ADuM7234 prepare to handle big gate capacitances?


I hope it was clear in my questions

Best regards