Question:

I'm reviewing a 24V DC input protection circuit and I'm trying to understand the design choice of using a bridge rectifier (DB207).


The 24V DC input comes directly from an SMPS, so the input polarity is already fixed and regulated.


The relevant part of the circuit is shown below.


My understanding of the components is:

• F1 – Input fuse
• MOV1 (MOV10D470K) – Surge protection
• BR1 (DB207) – Full bridge rectifier
• RV1 (2057-07-BLF) – Surge protection device (not sure if TVS/GDT)
• D1 (SMBJ36CA) – TVS diode
• FB1 – Ferrite bead
• C5, C6, C7 – Input filtering capacitors

My question is:

Since the input is 24V DC coming from an SMPS, the polarity should already be correct. Why is a full bridge rectifier still required?

Wouldn't a reverse-polarity protection MOSFET (or even a single diode) be sufficient if the only concern is accidental reverse wiring?


Is the bridge rectifier being used for one of the following reasons?

1. To protect against accidental reverse polarity during installation?
2. To make the board polarity-independent so the input wires can be connected either way?
3. To make the same PCB compatible with both AC and DC inputs?
4. Some other industrial design practice that I'm not aware of?

From what I can see, the bridge rectifier ensures the downstream circuitry always sees the correct polarity, but it also introduces a voltage drop of roughly 1.2–1.8V and additional power dissipation. Since the source is already a regulated 24V DC SMPS, I'm curious why the designer chose this approach instead of a lower-loss MOSFET-based reverse polarity protection circuit.


I'd appreciate any insights into the design rationale.

 

Hard to know for sure, but it's likely for one or more of the 1 through 3 reasons you listed.

Certainly if the only thing needing protection is reverse polarity, then a MOSFET circuit would be lower loss.

 

It’s not so it could take an AC input, otherwise there would be more capacitance.
As it’s a small bridge rectifier, I would assume the current is low enough that the power loss is insignificant. In which case it is for polarity protection, and the bridge was a cheaper choice (or just simpler to understand) than the MOSFET circuit.

 

Most of the components are there to prevent various transients and EMI from getting on the 24V output. RV1 is a 75 volt gas discharge tube. I suppose you could think of that component as akin to a neon lamp. When that fires, its impedance drops to almost nothing, so the transient energy must be absorbed by the total sum of series impedance before the device. That impedance will be the wiring and bridge rectifier. Notice the 100uF capacitor at the output. That is large enough for a low current supply, perhaps 100 mA or less.

Like others have said, "kind of hard to know", but based on the above, my take is that the bridge rectifier is there to make it a universal input: AC, DC+, DC-.

 

My experience has been that it is not possible to mark a connection adequately enough to prevent somebody from connecting the polarity backwards. Thus adding circuitry to allow the correct operation even with the reversal is a good choice. And the first time it prevents damage it becomes a very good choice.

 

Question:

1. To protect against accidental reverse polarity during installation?
2. To make the board polarity-independent so the input wires can be connected either way?
3. To make the same PCB compatible with both AC and DC inputs
4. ?

My personal experience I would say 1,2,&3 , AKA " steering Diodes".

 

I agree with MAX! All of the reasons given are valid.