HexFred vs. Hyperfast Diode (1200v)

[MelB]

I have some HexFred's:

http://www.vishay.com/docs/94038/hfa06tb1.pdf

and some Fairchild Hyperfast Diodes:

http://www.fairchildsemi.com/ds/RH/RHRP8120.pdf

I'm going to use 4 of them to make a bridge on a 356v @420mA winding. Any smart people here have an opinion or experience which would lean to one over the other?

 

[GoatGuy]

Yep... you want opinion, I got opinion.

Truly, to the core of the topic ... it makes NO difference. None. Not one iota. Zip. Zero. Zilch.

Why?

Because you're going to filter the baby Jesus out of your stream of rectified pulses, to get rid of ALL those consarned bumps. You're shooting for sweet, clean, nearly-ripple-free DC to apply to your input, mid, and output stages. DC is what you want, and DC is what you're designing for.

The "speed" of the (excuse me damned rectifiers will impact the sweet, clean, smooth DC not one bit. Certainly not after it is used to charge up a front-end capacitor, then an inductor, then another capacitor, then a regulator, then yet another capacitor.

Please don't let the (excuse me idiocy of abstruse discussions of flyback-voltage and restoring current, and back-EMF lead you astray. THEY DON'T MATTER.

GoatGuy

 

[Mark Johnson]

Maybe original poster MelB was thinking about the radio frequency interference that is generated when a rectifier diode shuts off. It's a well known phenomenon: the transformer secondary's inductance forms an LC resonant circuit with (the secondary's interwinding capacitance, plus the capacitance of the rectifier(s)). And it has a well known cure: install a carefully chosen resistance so the resulting RLC resonant circuit is damped and doesn't oscillate.

The phenomenon occurs in 50/60 Hz mains powered transformer+rectifier circuits, but it is much worse in 100 kHz switch mode power supplies. Capacitor vendor Cornell Dubilier provides a circuit design white paper on their website cde.com , telling customers about the problem and its solution
http://www.cde.com/tech/design.pdf

Consultant Jim Hagermann wrote an article detailing the calculations for a 50/60 Hz mains powered power supply, intended for audio applications
http://www.hagtech.com/pdf/snubber.pdf

As far as choosing rectifier diodes is concerned, the summary is: design the snubber properly, with a damping factor in the range 0.5 < zeta < 0.9, and you can use any diode you like. Fast recovery, ultra-fast recovery, slow recovery, silicon, Schottky, FRED, Soft recovery, hard recovery, low capacitance, high capacitance, whatever. Pick any rectifier diode you like; you can prevent it from generating RF noise by proper design of a snubber.

As the Hagermann paper shows, you'll need to know your transformer's secondary inductance and capacitance to design the snubber. Fortunately you can measure both of these quite easily. Just drive the secondary (primary open-circuited) with a signal generator and measure the self resonant frequency. That'll be the frequency at which the signal amplitude is greatest (or for those who remeber EE-101: the frequency at which phase shift is zero). Now attach an extra capacitor Cx whose value is known, and measure the new resonant frequency. Voila, two unknowns (Ls and Cs) and two equations. Apply a little algebra and boom yer done.

I prefer the phase measurement because it is much more sensitive, however it does require a 2 channel oscilloscope. One channel connected to the signal generator output, one channel connected to the transformer primary. Dial the frequency until the waveform zero crossings align. Takes 15 seconds.

 

[Triodethom]

Both are over kill for current 6 to 8 amps rated is much larger then you need. 1 amp or 3 amp uf type will do at less cost . Then follow The snubber advice from transistormarkj and GoatGuy

 

[Mark Johnson]

Whoops! the above post got an important detail backwards! It says

... drive the secondary (primary open-circuited) with a signal generator ...

But in fact you want to SHORT CIRCUIT the primary, so that you'll measure the secondary's leakage inductance. I typed it wrongly & apologize.

This is covered in Hagermann's white paper, page 5:

Leakage inductance can be determined by shorting the primary and measuring the inductance of the secondary (out of circuit, of course). ... Connect the output of a sinewave generator through a series resistor and the [transformer secondary, with primary shorted] . Measure the voltage across the [secondary] while varying the frequency. The lowest frequency which gives a peaked reading is probably the natural resonant frequency of the [secondary] .

You can experiment with this measurement technique in simulation. Schematic and example output are shown below.

 

[MelB]

I'm going with the Hexfreds. Seems the 6 amp version is the lowest amp rating one they make and at a whopping $7.00 CND for 4 of them I'll just use them straight up and feel good about the lower switching noise inside my amp. Weather or not it is making any difference to the sound appears to be a matter of shall we say debate.
Thanks for the feedback guys!
From the manufacturer if anyone is interested (could be biased):
In addition to ultrafast recovery time, the HEXFRED® product line features
extremely low values of peak recovery current (IRRM) and
does not exhibit any tendency to “snap-off” during the
tb portion of recovery. The HEXFRED features combine to
offer designers a rectifier with lower noise and significantly
lower switching losses in both the diode and the switching
transistor. These HEXFRED advantages can help to
significantly reduce snubbing, component count and
heatsink sizes. The HEXFRED VS-HFA06TB120... is ideally
suited for applications in power supplies

 

[canvas]

Whoops! the above post got an important detail backwards! It says



But in fact you want to SHORT CIRCUIT the primary, so that you'll measure the secondary's leakage inductance. I typed it wrongly & apologize.

This is covered in Hagermann's white paper, page 5:

Leakage inductance can be determined by shorting the primary and measuring the inductance of the secondary (out of circuit, of course). ... Connect the output of a sinewave generator through a series resistor and the [transformer secondary, with primary shorted] . Measure the voltage across the [secondary] while varying the frequency. The lowest frequency which gives a peaked reading is probably the natural resonant frequency of the [secondary] .

You can experiment with this measurement technique in simulation. Schematic and example output are shown below.

I read the Snubber article and have a question. The simulation was done without large filter capacitor. Shouldn't the Cx include the main filter capacitor we're using? To my understanding, Cx's location is where we put main filter capacitor, right?

 

[Mark Johnson]

I read the Snubber article and have a question. The simulation was done without large filter capacitor. Shouldn't the Cx include the main filter capacitor we're using? To my understanding, Cx's location is where we put main filter capacitor, right?

Transformer secondary -> rectifier(s)/bridge -> filter capacitor -> voltage regulator if used -> load

Oscillatory ringing occurs because the transformer's secondary leakage inductance resonates with the (transformer secondary's parasitic capacitance) and (the capacitance of the rectifier(s)). That's where the snubber needs to be installed, to damp out the ringing: across the transformer secondary. Before the rectifier(s).

Try it yourself in simulation; place the snubber into various different positions and see how effectively it works or doesn't work.

 

[canvas]

Transformer secondary -> rectifier(s)/bridge -> filter capacitor -> voltage regulator if used -> load

Oscillatory ringing occurs because the transformer's secondary leakage inductance resonates with the (transformer secondary's parasitic capacitance) and (the capacitance of the rectifier(s)). That's where the snubber needs to be installed, to damp out the ringing: across the transformer secondary. Before the rectifier(s).

Try it yourself in simulation; place the snubber into various different positions and see how effectively it works or doesn't work.

Thanks for the detail. I will definitely try that after I got my first LCR meter!
BTW, could you explain more about the measurement of the inter-winding capacitance? On Page 5, Line 6, the author stated that "connect the output of the generator through a series resister and the coil". What value would this resister be? And, 'the coil', I assume that would be the secondary, right?

 

[Mark Johnson]

. . .author stated that "connect the output of the generator through a series resister and the coil". What value would this resister be? And, 'the coil', I assume that would be the secondary, right?

Your post is #9 in this thread. If you look at ALL of the posts that came before yours (there are eight of them), and if you study EVERY single one of the attached figures connected to those eight prior messages, your questions will be answered.

 

[canvas]

My mistake. It's on your post#5. I thought you just paste the content, so I didn't pay attention to it.

 

[canvas]

Your post is #9 in this thread. If you look at ALL of the posts that came before yours (there are eight of them), and if you study EVERY single one of the attached figures connected to those eight prior messages, your questions will be answered.

transistormarkj,

I got another question. I use 4 diodes as bridge rectifier, how do I calculate the capacitance of the bridge rectifier? Let's say each diode has 50pf capacitance. Should I put 50x2=100pf into the equation? or, 50*4=200pf? Thanks again.