siliconchip.com.au Australia’s electronics magazine July 2019 27
Mosfet gate drive
To switch on an N-channel Mosfet, the gate needs to be
driven several volts above the source.
In this circuit, all the Mosfet sources are connected to-
gether and when the Mosfets are switched on, they will all
rise to the battery voltage – ie, around 12V.
Therefore, the gates need to be driven to at least 17V and
ideally higher, to 20V or more, to ensure that they switch
on fully and have the lowest possible resistance and dis-
sipation.
This voltage is generated by comparator IC1a, which is
configured as an astable oscillator and drives a charge pump.
The frequency of this oscillator is set to around 15kHz by
the combination of the 22kΩ feedback resistor and 3.3nF
timing capacitor.
Output pin 2 of IC1a is pulled high by a 4.7kΩ resistor,
and the resulting square wave causes the 100nF capacitor
to charge up to around 12V, via diode D2, when output
pin 2 goes low.
When that pin goes high, to around 12V, the anode of di-
ode D3 is lifted up to around 22V and this voltage in turn
charges the following 100nF capacitor which supplies the
Mosfet gates with about 20V via the following 10kΩ resistor.
That is, as long as output pin 14 of inverter IC1c is not
being held low. If it is, this shunts any current flowing
through that 10kΩ resistor to ground, holding the gates low.
At the same time, to save power, when pin 14 goes
low, diode D1 becomes forward-biased and this dis-
charges the 3.3nF timing capacitor, disabling the
oscillator which generates the gate drive voltage.
Zener diode ZD1 protects IC1 from supply spikes, in com-
bination with the 100Ω series resistor from the main bat-
tery, which limits the current through ZD1 should it con-
duct. Zener diode ZD2 protects Mosfets Q1-Q6 from dam-
age due to excessive gate voltages.
This is important as when the ~20V gate drive is initial-
ly applied, their sources are at 0V, and this could other-
wise exceed their maximum ±20V VGS ratings. However,
ZD2 will not conduct for long, as the source voltage will
quickly rise, reducing VGS to around 7-8V under steady
state conditions.
TVS1 and TVS2 are transient voltage suppressors, similar
to zener diodes but more robust. These protect the unit and
especially the Mosfets from high-voltage transients which
are common in the automotive environment.Construction
The prototype was built on two boards, with the control
circuitry on a piece of stripboard and the Mosfets, TVSs
and battery connectors soldered to a double-sided ‘blank’
PCB which was manually cut into large, isolated sections
of copper that the components were then soldered to.
You can also build it this way, and we will give some
information later on how to do so.
However, to make your life easier, we have produced two
commercial double-sided PCB designs. Again, one is for the
control circuitry and the other for the larger components.
You then just need to solder the components to these two
boards, join them and mount them in the case.
Fig.2 shows the control board while Fig.3 is the Mosfet
board overlay diagram. Use these and their matching pho-
tos as a guide during construction.
While the prototype had all six Mosfets on the same sideFeatures & specifications
- Suits most 12V batteries
- Waterproof
- Silent
- Solid-state (no relays)
- Easy construction and installation
- Switch-on voltage: 13.4V (13.13-13.67V*)
- Switch-off voltage: 12.6V (12.35-12.85V*)
- Quiescent current: approximately 3mA when off,
7mA when on - High current handling (>100A peak, >40A continuous)
- Low voltage drop: typically <1mV/A
Low dissipation: typically <1W @ 30A
*if some ±0.1% resistors are used (see parts list)
Fig.2: one of two PCBs in this project, the control board, with matching photo alongside. You could also build this on
stripboard if you wished (see page 30) but PCBs make a much neater job and minimise the chance of errors.