When powering a battery operated circuit, the experimenter must make a decision: either take his chances (Murphy loves this one), or opt to add protection in the form of a simple series diode. Unfortunately, as the voltages keep get lower and lower (e.g. 1.2V), a series diode ceases to be a valid option. The solution of choice is the proper application of a MOSFET as a synchronous rectifier–it is extremely simple, effective and inexpensive. A class of low voltage MOSFETs has been designed for this application. In this discussion, two tiny SO-23 MOSFETs are presented. Unfortunately, thru-hole devices are not available in this class so it is time for the experimenter to ‘bite the bullet’ and ‘get on board’ with SMD components.
Previous electroschematics articles for reference
How it works
The MOSFET device has an intrinsic body diode that is generally reverse biased so that it does not interfere with ‘normal’ operation. However, when the polarity is reversed, this diode conducts just like a normal silicon diode–in the case of inverters, this provides a necessary discharge path for reactive currents–if the MOSFET is gated on during this period, the current flows through the low Rdson resistance rather than the intrinsic diode–this is called “synchronous rectification.”
In the case of reverse battery protection, the current always flows in the synchronous mode, or backwards through the MOSFET device. When power is applied to the drain terminal, current flows through the body diode and raises the source terminal to the applied voltage minus one diode drop. Assuming high side protection with a P-channel device, this raises the voltage of the source in regard to the gate so the device turns on (a negative turn-on voltage is effectively applied to the gate). When the MOSFET turns on, the diode drop potential is eliminated so that source is at an even higher voltage–the gate now has essentially full battery potential across it and it and the device is fully turned on.
Should the battery be installed reverse, the body diode is reversed biased and no current can flow and there can be no gate to source voltage–fully protected!
High side or low side application
Obviously both high side and low side applications work well, so this is a judgment call, but there are numerous tradeoffs. The N-channel device (low side application) has lower Rdson and a lower price tag. On the other hand, some P-channel devices work at lower voltages (approx 0.2V lower), and that suits it well for 1.2V battery applications. The rest is human–the mind visualizes well against ground potential and it always wants common to be common and that tends to preclude low side application.
Testing an actual circuit
Not having the recommended devices on hand, I experimented with standard TO-220 MOSFETs in a 9V application with a 36mA load. For some unknown reason, calculated voltage drop was about half of what was actually measured, but was still very low. Results are as follows:
The case for rechargeable and /or keyed batteries
Reverse polarity protection may be omitted in the case of permanently installed rechargeable batteries or serviceable rechargeable batteries with physical keying. This includes many cameras and virtually all cell phones. This avoids the series resistance of the protection device that is more problematic in high power applications.
Recommended device specifications
P-Channel MOSFET: Vishay Siliconix Si2333DDS, Rdson = 0.028Ω, DigiKey SI2333DDS-GE3CT-ND, SO-23 Package, $0.50 each, Datasheet link
N-Channel MOSFET: Alpha & Omega AO3416, Rdson = 0.022Ω, DigiKey 785-1011-1-ND, SO-23 Package, $0.44 each, Datasheet link
Device characteristic curves
This works well. I recommend that all experiment with these circuits.
For the future
Compact, inexpensive low-voltage battery packs for the experimenter–new to the world…
Undocumented words and idioms for our ESL friends
bite the bullet –idiomatic expression –battlefield pain control, biting into a lead bullet while undergoing emergency surgery etc. –in engineering, it refers to doing something new or different that has been long put off
get on board –idiomatic expression –literally get onto the ship –in engineering it simply means to agree with the majority when it comes to task solving