transient voltage problem

Thread Starter

eb3484

Joined Dec 7, 2016
8
Hi,
I am driving TFT LCD backlight with 24 V smps adaptor. There are 6-series leds for backlight. At the same time i am driving a bldc motor, when the motor speed is increased there is a 34V transient voltage spikes. So, the spike causes light sparkling on the TFT LCD. For this i have used a 24V TVS diode, it clamps it a little but it did not work clearly, because i can still see the light spark on the screen when i increase the motor speed. I just want to learn whether a transistor-zener diode regulator circuit clamps the voltage spike clearly or not. Any advise ???
 

crutschow

Joined Mar 14, 2008
34,281
What circuit is driving the motor?
How much decoupling capacitance is connected across the motor terminals?
How much capacitance across the TFT and TFT driver power to ground?
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
For bldc motor i am using drv8305 drive ic, so according to drv8305 there is no decouple capacitors across motor terminals. For TFT LCD ,
it has in circuit driver, so i just use 100nF capacitors on VCC pins.
 

crutschow

Joined Mar 14, 2008
34,281
You should add at least 100μF across the drv8035 and 10μF across the LCD driver circuit power to ground pins.
Make sure that the motor power and return current from the motor do not go through the LCD circuit wiring.
The motor and drv8035 power and return should go directly to the power supply.
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
While driving bldc motor, there occurs voltage spikes when motor pwm decreased. I have atached the image below. By the way i am using DRV8307 EVB circuit for driving motor. What can be done for eliminating the voltage spikes? I have used a 24V TVS diode at the power supply output ( motor power supply input) but i did not work .

tek00000.png
 

drc_567

Joined Dec 29, 2008
1,156
... does the motor stator act as an inductor when the PWM decreases? Try placement of a reverse biased diode in parallel with the motor.
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
Did you not see my suggestions in post #4?
That only helps for negative spikes that an inductor would generate, not the positive spikes he is seeing.
Hi crutschow,
I could not understand what you mean by "100μF across the drv8035 and 10μF across the LCD driver circuit power to ground pins". it may be more helpfull for me to explain more clearly. Apologize for me, i have a little hardware information.
 

crutschow

Joined Mar 14, 2008
34,281
Hi crutschow,
I could not understand what you mean by "100μF across the drv8035 and 10μF across the LCD driver circuit power to ground pins". it may be more helpfull for me to explain more clearly. Apologize for me, i have a little hardware information.
The power pins are the pins that are connected to the power supply.
The ground pins are those connected to ground.
You connect the capacitors across those two points with short leads.
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
The power pins are the pins that are connected to the power supply.
The ground pins are those connected to ground.
You connect the capacitors across those two points with short leads.
Ok, there is already a 680uF/50V capacitor for across the VM pin of the DRV8307 and ground. Beside this, i have tried for 100uF capacitor for LCD power, but both of them did not work for the problem. There is still voltage spikes.
 

MrChips

Joined Oct 2, 2009
30,707
Ok, there is already a 680uF/50V capacitor for across the VM pin of the DRV8307 and ground. Beside this, i have tried for 100uF capacitor for LCD power, but both of them did not work for the problem. There is still voltage spikes.
Sometimes less is better.
Instead of 100μF, use 0.1μF in parallel with a 10μF capacitor.
I am not suggesting that this will fix the problem. I'm only suggesting that sometimes going too big is the wrong direction to take.

In your situation you may have to look at the complete wiring of the LCD supply, data and control lines.
How long are your connections? Show us some photos.
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
Sometimes less is better.
Instead of 100μF, use 0.1μF in parallel with a 10μF capacitor.
I am not suggesting that this will fix the problem. I'm only suggesting that sometimes going too big is the wrong direction to take.

In your situation you may have to look at the complete wiring of the LCD supply, data and control lines.
How long are your connections? Show us some photos.
I have attached my circuit schematic, 24V is used for motor drv8307 and tft lcd backligth supply. the LCD supply, data and control lines does not exceed 5-7cm length.

circuit.PNG
 

drc_567

Joined Dec 29, 2008
1,156
The drv8305 driver is listed as having slew rate control. Maybe a modification of that parameter could have the desired effect.
 

MrChips

Joined Oct 2, 2009
30,707
Can you provide us with more info on the TFT LCD module such as make and model number?
What is the purpose of R7 in the schematic?
What are the functions of P1 pin-3 and pin-4?
If these are power supply pins then you need the 0.1μF and 10μF parallel combo on pins 3 and 4.
Still waiting to see photos of all wiring connections.
 

Thread Starter

eb3484

Joined Dec 7, 2016
8
Can you provide us with more info on the TFT LCD module such as make and model number?
What is the purpose of R7 in the schematic?
What are the functions of P1 pin-3 and pin-4?
If these are power supply pins then you need the 0.1μF and 10μF parallel combo on pins 3 and 4.
Still waiting to see photos of all wiring connections.
The purpose of the R7 is for setting the LCD backlight supply voltage to 19.2V according to lcd specification. But it is value is 220R not 1K.
P1 pin3-4 are the pins of the TFT LCD backligth ANODE pins ( both of them are same). The TFT LCD model number is SAT035TM54DHR1-A0-01 (https://img.ozdisan.com/ETicaret_Dosya/366159_781471.pdf)
 

ronv

Joined Nov 12, 2008
3,770
For bldc motor i am using drv8305 drive ic, so according to drv8305 there is no decouple capacitors across motor terminals. For TFT LCD ,
it has in circuit driver, so i just use 100nF capacitors on VCC pins.
Try turning on diode freewheeling and off active freewheeling. This will prevent regen - I think.:rolleyes:
 

ronv

Joined Nov 12, 2008
3,770
Or power the LCD from the 24v directly (before the diode). You can add another diode if you like to protect the LCD from power reversal. In any case you need to make sure other circuitry can withstand the 36 volts. The voltage may go even higher if you reverse the motor before it has stopped.
 
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