2016/1/25 下午 06:47:27        Administrator        General   0 Comments
      For you are unfamiliar with the characteristics of MOS body diodes, chances are you will be bugged with voltage leaks, which, most commonly, will causes your system to boot up automatically due to erroneous sequencing, or not booting up at all.
Case study 1: Based on the circuit below, 5V_EXT is introduced early in the circuit, and will only be conducted for 5V when Q8 G is at Low after setting SLP_S3 at High. However, the body diode for Q8 PMOS is directed from D to S, leading to voltage leaks to +5V from 5V_EXT before SLP_S3 is being set to High and give rise to a sequencing error in the board.

Case study 2: The correct way to connect this circuit is, since 5V_EXT is introduced early, voltage leaks can be prevented as long as a reverse biased MOS body diode is used, correcting the sequence to allow 5V-EXT to send power to 5V after SLP_S3# is at High.

So, it is important to take note of the direction of the body diode when using MOS to prevent voltage leaks. This characteristic of body diodes can also be found in the datasheet of AP4435GM. 

(Image excerpted from APP4435GM Datasheet)

A Brief Introduction to MOS Body Diodes
In the case of NMOS, IC manufacturers will connect the B and S-side together for power MOS to prevent issues like body effects, but this will also create a MOS body diode (see image a).
      The reason for this is because both body and source, and body and drain, can be treated as states in a PN diode. As shown in the image below, (1) When the Source and Body are connected (similar to the red diode by the left), both the positive and negative end has the same voltage, negating the effect of the diode. (2) When the Souce and Drain connected, it can also be treated as a diode (similar to the red diode by the right). This is the MOS body diode.

Image excerpted from “Microelectronic circuits” by Sedra Smith

      When MOS is used as a power switch, most users might think it is non-directional and the D and S end can be used interchangeably. But in actuality the same cannot be applied to BS-connected Power MOS. For example:
Taking into consideration connecting the sorce to VDD and drain, we can see that:
  • VB=VS=VDD (negligible when same voltage exists at the left diode), VD =0, VBD = VDD
  • VBD forward bias (greater than cut-in voltage of the right diode)
  • BD of the diode is forwardly conducted
  • Load voltage for CL is VD=VS – cut-in voltage of the diode
  • The gate loses its regulative abilities, DS is treated as a diode (and causes voltages to leak)

Image excerpted from “Microelectronic circuits” by Sedra Smith

Example 2:
When the drain is connected to VDD, and the Source to the load, base don analysis, the BD ends can be treated as a reversed biased diode and nothing is conducted till a “route” is created by applying reverse voltage to the G-end.
  • VB=VS=VDD (negligible when same voltage exists at the left diode), VD-VDD, VBD=-VDD
  • VBD is reverse biased (the right diode is not conducted)
  • Not “route” is created till a voltage is applied at the gate (thus no voltage leaks)

Image excerpted from “Microelectronic circuits” by Sedra Smith

A BS-connected power MOS is directional. For NMOS, we will be connecting the Source (the pin nearer to the Body) to a low voltage source, while the Drain a high-voltage source to prevent a body diode from being conducted.
On the other hand, for PMOS, we will be connecting the Source (the pin nearer to the Body) to a high voltage source, while the Drain a low-voltage source. With a comprehension on the characteristics of MOS Body Diodes, its direction is noted during its design phase so as to avoid voltage leaks and upsetting the sequence during boot up.

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