The Tech Blog

You have all heard that Lithium polymer batteries are dangerous if not handled properly. It’s so because of its high energy density and the chemical/mechanical structure of the battery. You have to ensure that the operating specs are maintained all the time. ie) Cell voltage shouldn’t exceed an upper or a lower limit, the current draw is maintained within a limit to adhere to the temperature specs. All these are made possible by a Battery Management System (BMS).

The most common chip you use for protection is a small 6-pin chip known as DW01(Available for less than $0.02). Check the schematic images. It’s usually paired with a Dual back-to-back MOSFET arrangement that gets controlled by DW01. You usually get this as a single IC package In normal operation both these MOSFETs are ON. For large charge/discharge currents, just connect multiple of these paired MOSFETs in parallel or use a pair of large power MOSFETs. You have 2 MOSFETs instead of one, because current can flow into a battery and out of it. When the MOSFET is OFF to disconnect, there can still be current flowing via the body diode which is prevented with with the Dual MOSFET setup.

Lets assume a charger that charges post the nominal cell voltage of a battery, the MOSFET M2 is turned OFF and the battery is cutoff from the charger. It’s turned ON when the battery either self-discharges or its voltage drops below a limit via a load. For the over-discharge case, when the battery falls below say 2.5V or so, M1 turns OFF and the load is disconnected from the battery. Now it turns back only when a charger pulls the voltage above 3V. It also has a short circuit current detector to cut off when a temp short happens.

So use the small protection circuit whenever you use a bare Lipo or Li-Ion cell. It’s very cheap to implement or buy off the shelf to add to your cell. Please don’t use a cell without it. It’s simply not worth the risk. I only talked about single-cell protection, for battery packs there are a lot of considerations, that I can go into in the future if there is interest.

You would have heard this a countless number of times, “Schottky diodes are fast”. Ever stopped to wonder what is “fast” about them and why? Let’s explore that today

When a Silicon PN jn diode is forward-biased, current flows through it once the voltage is above the depletion layer potential. Now assume the case, where you suddenly remove the forward voltage and apply a negative voltage across it. We expect the diode to become reverse-biased and have no current flow. This occurs, but not as immediately as you think. There is a finite delay post the switching, where the current flowing across the junction goes negative(Or flows opposite). This occurs because when the diode is conducting, minority carriers (electrons in the P-type material and holes in the N-type material) accumulate near the junction. When the diode switches off, these minority carriers must return to their original regions or recombine. The accumulated charge in the junction decreases to zero. Now the current starts flowing in the opposite direction and this is known as the reverse recovery current. It goes up to a negative value and then slowly recombines back to a zero state when the diode becomes non-conducting. The delay time needed for it is called Reverse Recovery Time.

This delay causes power losses whenever a diode voltage switches so you always try to use diodes with low reverse recovery time. It becomes critical in switching power supplies with high switching rates. Schottky diodes don’t have this problem because it’s not a PN junction. It’s formed with a N-type material and a metal junction. Hence they have low switching loss and have a “fast” switching time with usually an order of magnitude faster times. So always check the switching times of diodes in the datasheet and use it to find the fastest edge switching rate in your application for the diode.