# Buck Converter Mosfet Vds distortion problem

#### Tommy Cheng

Joined Mar 5, 2016
6
Here is my schematic:

Vds when Vin = 12V

Vds when Vin = 20V

Vds when Vin = 30V

My switching frequency is 50kHz, duty cycle is 0.5
When Vin = 12V, the buck converter worked properly, but when I increased the input voltage from DC power source, the Vds got distortion, so why appear this problem? and How to solve it? Thank you very much.

#### ScottWang

Joined Aug 23, 2012
7,410
I'm not sure what you expect about the IR2104 chip:
Vcc Max= 20V
Vin = Vcc + 0.3

Please try the Vin = 16V and 18V.

#### Lestraveled

Joined May 19, 2014
1,946
Welcome to the forum. Your first post is a good one.

First, it looks like you are exceeding the ET rating of the inductor. Do you have a part number/data sheet for it?

#### Tommy Cheng

Joined Mar 5, 2016
6
I'm not sure what you expect about the IR2104 chip:
Vcc Max= 20V
Vin = Vcc + 0.3

Please try the Vin = 16V and 18V.
Nice to meet you ScottWang. Thank you for your reply. One additional point I want to add is that the magnitude of Vgs seems like increase when Vds got distortion so Vgs cannot keep constant right now. For IR2104, the range of Vcc is from 10-20V. That means 12V Vcc should be OK for it.

Welcome to the forum. Your first post is a good one.

First, it looks like you are exceeding the ET rating of the inductor. Do you have a part number/data sheet for it?
Nice to meet you Lestraveled. Thank you for your reply. For the inductor, I bought it from Taobao. I only got the information of core type, wire width,current rating and inductance. Can you explain what is ET? Can I test it in laboratory?

#### Lestraveled

Joined May 19, 2014
1,946
OK
ET = volts times micro-seconds. It is a way to understand when an inductor will start to saturate when voltage is applied to it. If ET is exceeded then the core saturates and the inductive reactance goes away and you are left with the DC resistance of the wire.

#### ScottWang

Joined Aug 23, 2012
7,410
Nice to meet you ScottWang. Thank you for your reply. One additional point I want to add is that the magnitude of Vgs seems like increase when Vds got distortion so Vgs cannot keep constant right now. For IR2104, the range of Vcc is from 10-20V. That means 12V Vcc should be OK for it.
If Vcc = 12V then the Vin =12V should be ok for your application.

#### Tommy Cheng

Joined Mar 5, 2016
6
OK
ET = volts times micro-seconds. It is a way to understand when an inductor will start to saturate when voltage is applied to it. If ET is exceeded then the core saturates and the inductive reactance goes away and you are left with the DC resistance of the wire.
I have done inductor calculcation based on Vin = 60-140V,Vout = 48V, Iout = 2.7A. 1mH inductance is already higher than the value I calculcated so inductance should not be the cause of saturation. And ET sounds like the switching time limit of inductor?

#### Tommy Cheng

Joined Mar 5, 2016
6
I have tried different kinds of inductor such as 100uH and 47 uH but it seems like it cannot not help... Maybe the problem is from Mosfet and/or bootstrap ?

#### Tommy Cheng

Joined Mar 5, 2016
6
OK
ET = volts times micro-seconds. It is a way to understand when an inductor will start to saturate when voltage is applied to it. If ET is exceeded then the core saturates and the inductive reactance goes away and you are left with the DC resistance of the wire.
If Vcc = 12V then the Vin =12V should be ok for your application.
I solved my problem !!!!!!!!!!!!!!!!! I found out MUR860 is the cause of Vds distortion. I replaced it using 1N4007.Then, there is no such a Vds distortion problem although the ringing problem is serious. Really thank you for your support!!

#### ScottWang

Joined Aug 23, 2012
7,410
I solved my problem !!!!!!!!!!!!!!!!! I found out MUR860 is the cause of Vds distortion. I replaced it using 1N4007.Then, there is no such a Vds distortion problem although the ringing problem is serious. Really thank you for your support!!
Great !
Could you try to increase the Vcc to 14V, the Vin = 12V and try it again?
.

#### Tommy Cheng

Joined Mar 5, 2016
6
Great !
Could you try to increase the Vcc to 14V, the Vin = 12V and try it again?
.
I have tried to increase Vcc to 14V, Vgs just increases the magnitude.

But right now I have two problems:
1. Vgs increases when Vin increases. It is serious because mosfet withstands 20-30V typically. I don't know why it happan?
2. When Vin=30V, Vds voltage ringing reaches 150V... But my design is for at least 60Vin. How to reduce the ringing?

#### ScottWang

Joined Aug 23, 2012
7,410
I have tried to increase Vcc to 14V, Vgs just increases the magnitude.
The Vcc is for the IR2104.

ut right now I have two problems:
1. Vgs increases when Vin increases. It is serious because mosfet withstands 20-30V typically. I don't know why it happan?
2. When Vin=30V, Vds voltage ringing reaches 150V... But my design is for at least 60Vin. How to reduce the ringing?
In parallel a resistor with coil, start from 10K and adjust it.