Do PWM solar controllers increase current?

Question

There are two kinds of solar charge controllers.

• PWM does simple pulse width modulation to limit intake voltage (E.g. 18V) to that appropriate for battery charging (e.g. 14V). The only thing it considers is correct battery voltage.
• MPPT controllers also "scan" the panel dynamically, varying their current draw to seek maximum watts. This is constructive because solar panels are not linear, and don't naturally seek watt maxima. Decreasing current draw 5% may increase voltage 15%, and that's a win. This MPPT strategy can get 30-40% more power out of a panel.

What I want to know is, does the PWM naturally increase output current when it decreases output voltage, I.e. does it act like a buck converter? Or must the excess voltage be consumed as heat in the PWM charge controller, like an LM7805?

If my Panel input is 14A @18V, is it possible for the PWM output to be 16A @14V? Or is it impossible for amps out to exceed amps in?

I mention MPPT because folk wisdom in solar is PWM controllers act like 7805s, and you need an MPPT to get the buck conversion/current multiplication effect. That advice sounds off.

Answer 1

Just the term PWM does not give enough information, it needs to be more explicit.

Some solar controllers use PWM but only to control the average power charging the battery, they do not attempt to store any energy during the cycle and are lossy

Some of those operate by shorting the PV array so the unwanted power is dissipated within the array. Others may disconnect the array in which case the array does not output any power when not required.

MPPT controllers use a DC-DC converter which itself uses PWM together with energy storage elements (inductors) to avoid energy loss. The energy is stored during part of the PWM cycle and then sent to the battery during the second part.

The average output current, in this case, can be more than the average input current but the power output cannot be more than the power input. Efficiency can be up to the high 90% region.

Answer 2

PV PWM charge controllers don't perform voltage transformation, they just supply current during the PWM on time(% of PWM period). Thousands of pulses per second.

"Simple" Pulse Width Modulation of the current from a Photo Voltaic panel can achieve efficient power conversion at MPP (matched impedance) into a resistive load if suitable capacitance is connected in parallel with the PV panel & PWM is appropriately controlled. This is due to the capacitor is charged during PWM off time from PV current & during PWM on time current from PV + Capacitor are supplied to load.

This is easy to test by applying a load with lower impedance than the PV panel(DC fan or Filament lamp etc...). Using a simple motor PWM board & capacitance on input (eg 1000uF/5khz/5A) adjust the PWM for maximum output & measure PV Volt & Current.
Example: ___ 100W 18Vmp panel, 100W filament 12V autolamp.

Below is a power plot of data acquired from PWM sweep from 100% --> 0% with 250W old panel & 1.5R load. A PIC16Fxx provided the PWM. The horizontal axis is the linear 1024point PWM sweep.

At MPP the effective load resistance(calculated) is 3.29ohm, data recorded 26.06V, 7.92A, 206.4W. @51% PWM. point 520 on graph. (conditions not STC).

I have a modified firmware solarPWM controller doing MPP tracking for my 30V HWS, no inductors (except the cabling), no cold showers for me.

Answer 3

No sane designer would implement a 7805-like voltage stepdown when trying to harvest max power from a panel, they'd always use a switch mode device, often a buck converter.

An ideal converter has the same power out as power in, so if the voltage is reduced, the current is increased. A practical buck will deliver an efficiency in the 80s to 90s percent.

Answer 4

As tonigau said, you can make a simple MPPT with capacitors in parallel with the PV panel and PWM is controlled based on the feedback of average voltage. Only monitor average voltage should be good enough, because most solar panel has certain MPPT voltage.

It's actually a step-down converter without a inductor if you look at this way: decrease the average voltage and increase the average current.

Answer 5

Using an LDO is ineffective for a current sourced PV unless perfectly impedance matched between source and load.

Using PWM does not match naturally impedances and thus cannot achieve maximum power transfer (MPT). It must using a reactive L impedance in a continuous mode to store the energy at a matched impedance. The PV impedance is known from the slope peak operating points which drop in current faster than voltage, thus rising Req.

A PWM modulates the impedance of a load by Z/d with duty cycle d (0 to 1)

Your load is between a current source PV with a computed MPT ESR of 18V/14A = 1.4 Ohms

Your battery ESR is low (tbd) and depends on C charge rate max.

If only pulsing this loop resistance , it is possible to regulate the open circuit voltage Voc down to 18V but with a certain power loss in the net ESR of all parts including PV load cap.

A more efficient solution uses a flyback series reactor of inductance L (Buck) such that when switched at a fundamental frequency of f creates an impedance of 1.4 Ohms minimum and rises with changing duty cycle from 50% as the fundamental voltage at f reduces and relative harmonic energy increases from asymmetry of pulse. (Fourier spectrum) yet avg. power increases with d.

Thus it does not present a perfect matched impedance at MPT to the source PV impedance to satisfy the ideal maximum power transfer theorem.

By choosing f and L with variable d for PWM, one can sense the source voltage output from PV and load current I to compute this impedance on a cycle-cycle basis or an average basis depending on what control Design is used. a flyback Zener is suggested to make switching off faster but lossy, or a Schottky diode for CCM mode or a Dual FET half bridge for Ideal diode characteristics.

Since the output battery charge must be regulated for CC and CV and cutoff, there must be a designed control mechanism for input MPT current and output battery CC current with Voltage regulation now on both LV and battery.

Now you have an MPT and battery charge controller in one system design.

A similar analysis may be done in the time domain rather than my frequency -domain matched-impedance analysis that I did. This may be Continuous or Discontinuous using CCM and DCM modes of switched reactor with amp-seconds of current transferred from input to output using LTI formula of V=LdI/dt for some switched interval ON and Off via the other conductor ( diode or FET).

You may try to understand this logic then search for simpler design rules to choose a PWM flyback PMT Battery charger or consider the losses in Efficiency from not achieving a matched impedance which rises as solar power input declines. Zpv = Voc/Isc *Pmax/Pin for some power ratio or “solarity” of PV input power. This is my approximation of a PV current source. ( for better or worse, it works)