PWM

[Neophyt]

If two PWM controllers are placed in parallel, is the output simply the sum of the on-times?

 

[kerberos_20]

in parallel voltage doesnt change, but current adds up

 

[Neophyt]

in parallel voltage doesnt change, but current adds up

During on-times, voltage is always max (except for a possible ramp), and current doesn't "add up." During on-time, current is constant (except for short transients, and possibe ramp time), and zero otherwise; average current depends upon on-time fraction. So, if two PWM controls are in parallel, if one or the other or both are "on" then voltage is Vmax, and constant current flows for the duration of the on-time. So, it seems to me, the result of parallel PWM/s should be an "on" when one or the other is "on" and off otherwise. Do you agree? Have I missed anything?

 

[kerberos_20]

During on-times, voltage is always max (except for a possible ramp), and current doesn't "add up." During on-time, current is constant (except for short transients, and possibe ramp time), and zero otherwise; average current depends upon on-time fraction. So, if two PWM controls are in parallel, if one or the other or both are "on" then voltage is Vmax, and constant current flows for the duration of the on-time. So, it seems to me, the result of parallel PWM/s should be an "on" when one or the other is "on" and off otherwise. Do you agree? Have I missed anything?

voltage depends on duty cycle, if you have 24V input with 50% duty cycle, then you will get 12V output, when its off, its disconnected = no current
if its for something like solar charger, you should expect pwm to be always on, it will start disconnecting solar to protect battery when batery is close to 100%

 

[kerberos_20]

lemme rephrase it a little bit
say you have two 24V 5A pwm controllers, each of them produces 120watts

you connect them in serial
output will be 48V 5A = 240watts
you connect them in parallel
output will be 24V 10A = 240watts

since they are PWM meaning you can lower its duty cycle from its normal operation
75% duty cycle:
serial = 36V/5A = 180watts
parallel = 18V/10A = 180watts
50% duty cycle:
serial 24V/5A = 120watts
parallel = 12V/10A = 120watts

hope this helps

 

[Grobe]

OP: Does the two PWM controllers have a complete schematic for the internal circuit ?

I ask because there are some issues you may run into:

Parallel connection of output

The basic function of the controller circuit is that there is a feedback of output voltage that the regulator use to control the output voltage. Dependent of the sensitivity of the feedback/regulator and the exact voltage at zero load, there is a risk that one of the converters take the most part of the load while the other always runs mostly idling.

Also - if this is some cheap product made in CHN, it may not deal with - being designed to work - if a higher voltage than the self no-load voltage is applied to the output terminals. What happens then? Well, have a look at the schematic. Most often, there is a diode for this in the very output stage, but if the manufacturer have other ideas, who knows what will happens?

Serial connection of outputs

Should work, but I have seen solutions where manufacturers have added resistor bridges at input stages, probably as a defense mechanism against over voltage. The issue may be when those resistor (very high values = very low currents) are being fed back to the regulator. Depending of the actual circuit design, there is a slightly chance that this may interfere with the voltage regulation so that you ending up not getting the output voltage that you expect.

 

[Neophyt]

OP: Does the two PWM controllers have a complete schematic for the internal circuit ?

I ask because there are some issues you may run into:

Parallel connection of output

The basic function of the controller circuit is that there is a feedback of output voltage that the regulator use to control the output voltage. Dependent of the sensitivity of the feedback/regulator and the exact voltage at zero load, there is a risk that one of the converters take the most part of the load while the other always runs mostly idling.

Also - if this is some cheap product made in CHN, it may not deal with - being designed to work - if a higher voltage than the self no-load voltage is applied to the output terminals. What happens then? Well, have a look at the schematic. Most often, there is a diode for this in the very output stage, but if the manufacturer have other ideas, who knows what will happens?

Serial connection of outputs

Should work, but I have seen solutions where manufacturers have added resistor bridges at input stages, probably as a defense mechanism against over voltage. The issue may be when those resistor (very high values = very low currents) are being fed back to the regulator. Depending of the actual circuit design, there is a slightly chance that this may interfere with the voltage regulation so that you ending up not getting the output voltage that you expect.

Thanks Grobe. I don't have a schematic for the device I have, so there probably isn't much use in pursuing this further.