I am currently developing a small low power (high switching) Buck converter and have run into a strange issue that I have never seen before and was looking for some insight into the issue. I am also controlling the FETS from an MCU, so I can set up arbitrary dead-times ect.
On the falling edge of the lowside gate, the high side immediately turns on. (As shown below) The first jump in the yellow trace is the HS turning on from the falling edge of the LS signal, while the second jump is the actual HS gate signal. This is regardless of the load I place on Vout.
*EDIT I have revised the image such that it shows the SW node and the Voltage on the gate. The reason I know that it is turning on is that I am getting ~25mA current draw through the main supply signifying a shoot-through occuring. This is again without any load on the IC. There should be ~.6V when the low side turns off since the inductor is causing the diode to forward bias.
I have tried a few different gate drivers that I have available, but they are all acting like this.
I assume that this is a layout issue, but I am not sure exactly what is going on to cause this to happen. I know that it has something to do with the FET input capacitance, but I don't really know how to measure/improve.
- Increased gate resistors to slow down switching
- Placed pull down resistors on gates (HS to SW node)
- Tried clamping SW point to HS gate with shotcky diode
As for the layout (this was a prototype board to test out a couple of ideas) I am severally constrained in terms of space to ~1" width on the board and 2 layers (as this is from a local protoboard shop).
Thus the FET driver is located on the opposite side as the FETs, but I have done as much as possible to lower the inductance (multiple VIAs, large traces, ect)
I should also note that there is a throughhole decoupling cap across the GateDrviers VCC/COM that is not shown here. Also there is a LS gate resistor that is not shown in the image.
I am quite puzzled over this, hoping that someone has seen this issue before and can lend some insight.
EDIT 1 To anwser some questions:
1) I have posted another scope print out that shows the SW voltage. The HS is coming on when the LS falls. I can also see this through the current (~25mA @17VDC, also the current draw is directly related to the input voltage...)
2) I am using a MCU to control the PWM, so there is no controller. The MOSFETS are BUK7Y153-100E.