The PWM controllers are running (switching) at 50 KHz and up, like a switching power supply without the inductor. The solar panel is performing the role of the inductor. In a normal buck converter when the switch closes, the inductor controls the rate of rise of the current. With these PWM controllers, the constant current nature of the solar panel performs the current control.

 

Because the panels are constant current devices and there is a good amount of capacitance at the input of the PWM switch.



No, what a PWM is actually doing is controlling the panels output voltage by shunting it with the battery. In other words, the PWM is using the battery to control the panel output voltage. You can call it what you like. A PWM charge controller does not allow the panel to operate at its maximum power output voltage. It forces the panel to operate at the voltage needed to charge the battery.

That's a simplistic way of looking at it. A proper shunt regulator provides an alternative path for current instead of the battery/load to regulate the output voltage. That not what's happening inside the Xantrex PWM controller. With series PWM the panel/battery voltage differential is the integrated panel voltage over the PWM cycle with the battery (Batteries make good filters). If the charger is in bulk charge mode and the charging current to the battery drops the panels voltage below the controllers max bulk voltage set-point the PWM circuit is ON close to 100% so effectively it's just a toggle switch direct connection to the battery. In this mode if the battery load and the panels IV power curve are closely matched (panels designed to charge at a set battery voltage) the losses from a PWM controller at a few hundred watts are usually in the 15% range of what a MPPT unit can do by optimizing the battery current in real systems. For MPPT systems charging batteries extra bulk mode current is the main advantage.

If the battery current draw is less than what's required to keep the battery voltage below the controllers bulk voltage set-point, the PWM ON period will start to reduce below 100%. This causes the integrated voltages in a PWM controller to differ between panel and battery. This also usually stops the MPPT controller from maintaining max power so it then uses a PWM type circuit to reduce voltage to the battery or adjusts the MPPT DC converter control loop for voltage regulation mode instead of optimize power mode.

The bottom trace (top is a signal gate) is from a Hall sensor measuring battery current from a C40 PWM controller limiting voltage by controlling the PWM duty cycle with a nearly charged battery.

Simple PWM charge controllers like the old C35/C40's run at a few hundred Hz.

We see a steep increase in current as the PWM switch turns on the battery to full panel voltage, a quick drop in current as the panel voltage quickly drops from the increased battery load with current leveling off due to nearly charged battery chemistry and then off to repeat the cycle.

 

@nsaspook
I yield to your to your excellent description. There is a vast difference between reading a few white papers and making assumptions verses having a PWM on your bench with a scope probe in hand.

Edit: One good experiment is worth more than 100 opinions.

 

OK, please excuse me, it is soap box time.

The TS started this thread with an EBay ad for a PWM charge controller with MPPT. This got me spun up because I feel it is miss-labeled as an MPPT. MPPT is a algorithm, that is paired with a high efficiency switching power converter. In a correctly sized system, the MPPT function is only on line when the system is starved for power, namely morning and late afternoon. When the panels are in full illumination, the MPPT function is disabled and the switching power converter is the major contributor to the efficiency factor. The major player is the high efficiency switching converter, not the MPPT algorithm, yet the two are paired together and called MPPT.

On this forum, switching power converters are often suggested to people building small solar chargers. Would you call a Texas Instruments "simple switcher" an MPPT charger? No, but some unscrupulous sellers would.

How about implementing an MPPT function with a PWM controller? OK, it could be done, but why? MPPT requires a microcontroller (and programming) to control a semi-inefficient PWM controller. The improvement made by MPPT in a PWM controller is not worth the added cost. But then again, did the designer actually add the MPPT function or just add the MPPT label?

This is my message: unscrupulous sellers will label anything with MPPT to get it to sell.

 

OK, please excuse me, it is soap box time.

This is my message: unscrupulous sellers will label anything with MPPT to get it to sell.

+1

I've no idea what controller the OP was talking about but many of the cheap EBay units are fakes.

 

Here the website got mppt+Pwm device from http://stores.ebay.com/kjfashion2011/_i.html?_nkw=mppt&submit=Search&_sid=1049494291

OK, please excuse me, it is soap box time.
Lestraveled what is(TS)mean.
I am mrel I started this thread asking can you have both MPPT +PWM in one device.
Do you know if there is Pure MPPT device without Pwm circuit?
I read somewhere MppT device have circuit that would boost current when the solar panel do not produce enough current ,not enough sun that day..
Since purchase this device from china will take three week before delivery.
When device do come i might be asking more questions.
Which device better to have MPPT or PWM to charge battery from solar power?
mrel
The TS started this thread with an EBay ad for a PWM charge controller with MPPT. This got me spun up because I feel it is miss-labeled as an MPPT. MPPT is a algorithm, that is paired with a high efficiency switching power converter. In a correctly sized system, the MPPT function is only on line when the system is starved for power, namely morning and late afternoon. When the panels are in full illumination, the MPPT function is disabled and the switching power converter is the major contributor to the efficiency factor. The major player is the high efficiency switching converter, not the MPPT algorithm, yet the two are paired together and called MPPT.

On this forum, switching power converters are often suggested to people building small solar chargers. Would you call a Texas Instruments "simple switcher" an MPPT charger? No, but some unscrupulous sellers would.

How about implementing an MPPT function with a PWM controller? OK, it could be done, but why? MPPT requires a microcontroller (and programming) to control a semi-inefficient PWM controller. The improvement made by MPPT in a PWM controller is not worth the added cost. But then again, did the designer actually add the MPPT function or just add the MPPT label?

This is my message: unscrupulous sellers will label anything with MPPT to get it to sell.

 

Here the website got mppt+Pwm device from http://stores.ebay.com/kjfashion2011/_i.html?_nkw=mppt&submit=Search&_sid=1049494291

This line == likely FAKE. "MPPT+PWM charging mode（Controller Provided MPPT Maximum power point tracking Function,charging mode is PWM)"
http://www.solar-electric.com/mppt-solar-charge-controllers.html/

 

Needed to make some hot weather (~100°) adjustments on the charging voltage.

PWM waveforms from my C40 charge controller adjusting for absorption voltage limits. At this point in the charging cycle any MPPT possible power gains are meaningless as the battery can't accept more current without raising the charging voltage above the recommended limits for flooded cells. MPPT could have shortened the time it took to get from completely discharged to this point by maybe 10-15 percent over the entire bulk, absorption, float cycle.

Scope traces
Top: Panel voltage at input terminal
Bottom: Battery voltage at output terminal. The negative going spikes from the several foot battery wire inductance when the ~30A current switches off and the magnetic field collapses is a major reason to keep the PWM switching frequency low.

Battery charging voltage is lower due to temperature adjustments in the controller.

Float