I am looking at a schematic for a solar charge controller: http://freechargecontroller.org/images/a/ab/Charge_controller_4_04b.pdf

My question is about the MOSFETS M1 and M2. The data sheet for the MOSFETS I am attaching: http://www.jameco.com/Jameco/Products/ProdDS/669943IR.pdf

I think I understand that there is a blocking diode inside each MOSFET that allows for the flow of electrons from the source to the drain but not in the reverse direction and that diode is called a body diode. It is shown in the data sheet for the MOSFET. I am looking at the way the source pin of M2 is connected and it appears to me that what is connected to the source pin of M2 is a conduit that runs between that positive terminal of the battery and the source pin of M2. There is an inductor coil on that conduit. Hopefully I will be corrected if this is not exactly the case. It appears that J2-2 also ties into the same conduit and there is a blocking diode D3 in place there.

If the source pin of M2 then has a positive charge and there is a blocking diode inside M2 then wouldn't that blocking diode in M2 prevent the flow of electrons from traveling from the drain of M2 onto the VS line when M1 and M2 are activated?

Also I think I understand that diode D2 and capacitor C2 are providing a bootstrap voltage but I'm not really sure yet how that bootstrap voltage is generated exactly or what it is exactly used for. Could someone explain?

Thank You


2 Answers 2


This is basically a buck regulator. M2 is the high side Nmos and M3 is the low side Nmos. The IR2104 is a driver chip, which takes care of the sequencing of the Fets. C2 is a boostrap cap that the IR chip uses to overdrive the gate of M2 so that it can enable.

M2 will only turn on when C6/C9 are below the regulator setpoint voltage. When the output voltage exceeds that threshold, M2 will disable and M3 will be enabled. M2 and M3 cannot be enabled simultaneously as they will short circuit.

The body diodes are a useful property as they provide some level of spike suppression.

You can read up on buck regulator basics, like the link below.



M2 and M3 form a half H bridge to drive a buck DC to DC converter. L1 and C6 form the LC low pass smoothing filter in this converter. Since M1, M2, and M3 are all N channel MOSFETs, their gate terminals must be pulled higher than the source voltage for them to turn on. This is not a problem for M3 as its source voltage is ground. However, this is a problem for M1 and M2 as their source voltages can be as high as the supply voltage.

The body diode in a MOSFET is a parisitic diode. It is a parisitic because when you make a MOSFET the way those are built, you end up geting an extra diode for free whether you want it or not. M1 is required to prevent the battery from discharging through the body diode in M2 when the solar panel voltage is lower than the battery voltage. The body diodes in M1 and M2 will be back-to-back, preventing current flow. R1 will turn off M1 by pulling the gate to the source. However, when the PWM signal is applied to the MOSFET driver IC2, M1 will be turned on through diode D1.

The bootstrap voltage is quite simple. Since the DC to DC converter is driven by a pulse width modulated signal, M2 and M3 are alternately opening and closing. This means that the voltage on the common node oscillates between ground and the solar panel output voltage. When this voltage is zero, C2 charges through D2. C2 will charge to the solar panel output voltage. Then the MOSFET driver grounds LO and connects HO to VB. The voltage on C2 turns on M2. Current flows through M2 and increases the voltage on pin 1 of L1. However, since C2 is now floating wrt. ground, M2 stays on as long as there is enough charge in C2. The charge in C2 will also be used to keep M1 on as long as the converter is running. The gate capacitance of M1 should keep it on when the MOSFET driver switches M2 off.

Note that this bootstrap method only works because this circuit is designed to be driven with a pulse width modulated square wave of several kHz. It does not work with a static high/low signal as C2 will eventually discharge.


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