Help with first Power Supply for LM1875

[Scratchy]

Originally posted by pacificblue

That changes nothing about the above explanation. This circuit is called "economy circuit" here in Germany. It has the same output like two full-wave rectified secondaries, but at the price of (slightly) lower efficiency. And it increases the probability of hum.

There is nothing wrong with this circuit, but it is only full-wave rectified, if you look at the entire secondary, not if you look at each half of the secondary. The difference in performance is small, but there.

Thanks for the detailed explanation pacificblue
That's much easier to understand than "No" or "Where's the apology" or "ridiculous proposition".

As per my earlier quote from the sound.westhost.com site:

Full winding utilisation means that the AC current is now the same as for a bridge rectifier, at 1.8 times the DC current, but only for a common mode load (i.e. between the supplies, rather than from one supply or the other to ground - this is identical to equal current from each supply to ground). Where the load is from only one supply or the other, the 1.2 rule applies, but power amplifiers will draw from both supplies (more or less) equally.

the difference in efficiency is indeed dependent on the whether the load is balanced or unbalanced.
Keep in mind when using 1 bridge per winding, there are double the number of diodes (twice the voltage drop) in the rail to rail path, which may eliminate any gains in efficiency over using a single bridge in the "economy circuit" layout.

In my case I am building this PS circuit to power a bridged amp, and I was trying to understand what benefit there was to using 2 rectifiers, rather than one. In my case it is better to use just one.

[whubbard]

I was just browsing around, and I noticed that the OP was having trouble with heat. My biggest suggestion would be to use very good thermal grease. It's not that expensive, but it will do a much better job getting the heat from the IC to the Heatsink. It doesn't change the heat dispensation requirement for the heatsink, but rather lets the heatsink do its job.

This stuff is basically the best. I mainly use it for computers, where thermal grease is VERY important, but it will do a fine job for IC->Heatsink connections.

Arctic Silver 5

-West

[Jeb-D.]

Originally Posted by pacificblue

The reason is that in the full-wave-circuit current is only flowing through each winding during one half-cycle, while with bridge rectification current flows during both half-cycles.

Thanks for the link, but I am having trouble understanding how this applies for bi-polar supplies. Using the single bridge config (aka dual full-wave). One winding is supplying positive cycle, while the other winding is simultaneously supplying the negative cycle. The direction switches for each half cycle, but current flows through each winding for the entire cycle.

[AndrewT]

Originally Posted by Jeb-D.

Using the single bridge config (aka dual full-wave). One winding is supplying positive cycle, while the other winding is simultaneously supplying the negative cycle. The direction switches for each half cycle, but current flows through each winding for the entire cycle.

I don't think you are explaining this correctly.
The whole winding flows in one direction and passes current through two of the diodes to the capacitor pair. If the +ve and -ve capacitors (plus the load currents if any) pass identical currents then, the centre tap passes exactly zero current.
During the other halfwave, the whole winding flows in the opposite direction and the other two diodes steer the current through the capacitor pair in the same direction.

The centre tap allows the loads to draw differing currents from the +ve and -ve halves of the capacitor pair. The centre tap passes the DIFFERENCE in output currents from the PSU.
If there is ONE load from most +ve to most -ve then the +ve & -ve currents MUST be IDENTICAL. With this single load situation the centre tap is not required and can be left open circuit.

[Jeb-D.]

Originally Posted by AndrewT

I don't think you are explaining this correctly.
The whole winding flows in one direction and passes current through two of the diodes to the capacitor pair. If the +ve and -ve capacitors (plus the load currents if any) pass identical currents then, the centre tap passes exactly zero current.
During the other halfwave, the whole winding flows in the opposite direction and the other two diodes steer the current through the capacitor pair in the same direction.

The centre tap allows the loads to draw differing currents from the +ve and -ve halves of the capacitor pair. The centre tap passes the DIFFERENCE in output currents from the PSU.
If there is ONE load from most +ve to most -ve then the +ve & -ve currents MUST be IDENTICAL. With this single load situation the centre tap is not required and can be left open circuit.

I agree with your explanation of operation, but still fail to understand why dual bridges are preferred over a single bridge. The entire winding is utilized in either configuration.

[sasmit]

I think a single bridge rectifier will also do, but guess the only reason dual bridges are used is to reduce the ammount of current through each bridge rectifier ...thus getting away with a smaller i.e. cheaper rectifier.
I myself use a dual bridge rectifier..but haven't done any R&D into finding if the dual bridge makes any difference.

[danielwritesbac]

Originally Posted by whubbard

I was just browsing around, and I noticed that the OP was having trouble with heat. My biggest suggestion would be to use very good thermal grease. It's not that expensive, but it will do a much better job getting the heat from the IC to the Heatsink. It doesn't change the heat dispensation requirement for the heatsink, but rather lets the heatsink do its job.

This stuff is basically the best. I mainly use it for computers, where thermal grease is VERY important, but it will do a fine job for IC->Heatsink connections.

Arctic Silver 5

-West

Danger of shock. Please substitute Arctic Ceramique instead of the shock.

[pacificblue]

Originally Posted by Jeb-D.

I agree with your explanation of operation, but still fail to understand why dual bridges are preferred over a single bridge. The entire winding is utilized in either configuration.

In a centre-tapped transformer current will only flow in one half of the winding during one half-cycle. If you use a dual secondary tranformer with full-wave rectification, i. e. one rectifier bridge per secondary, current will flow in each secondary during both half-cycles. That leads to a higher efficiency for the dual secondary version.

[AndrewT]

Pacific,
I think you are not helping us understand by omitting to clarify which output currents you are referring to.
Try again.

[pacificblue]

I refer to the current that flows from the transfromer secondaries through rectifiers and amplifier to the speaker. I skip the quiescent currents, bias currents, etc.

A single amplifier channel will only draw current from one rail at a time. We can therefore limit us to the components that are active to supply one rail. If we look e. g. only at the positive rail, the centre-tapped transformer with two diodes forms, what my dictionary translates as midpoint connection.




When the amplifier draws current, during the positive half-wave D1 conducts and current flows only in the upper half of the winding. During the negative half-wave D2 conducts and current flows only in the lower half of the winding.



A single secondary with four diodes or a Graetz bridge translates to bridge rectification.



When the amplifier draws current, during the positive half-wave D1 and D4 conduct. During the negative half-wave D2 and D3 conduct. In both cases current flows through the upper winding. Among other effects the difference in the resulting wave-form through the windings leads to a difference in the effective current that loads the primaries.

An old textbook of mine contains a table that states the relations between transformer power Pt and DC power Pdc as follows.

Midpoint connection: Pt / Pdc = 1,5
Bridge rectification: Pt / Pdc = 1,23

In other words for 100 W power consumption you need a 123 VA dual secondaries transformer with two rectifier bridges or a 150 VA dual secondaries or centre-tapped transformer with a single rectifier bridge. 100 W power consumption won't give more than 60..70 W of continuous amplifier output power from a class AB amplifier.

A dual secondaries transformer with two bridges must be rated twice as big as the desired output power, while the centre-tapped or dual secondaries transformer with a single bridge requires a rating 2,5 times as big as the desired output power. If you want continuous maximum output power, that is.