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How to calculate UC3842 components?
technix:
I am in the middle of designing M103v2 power supply, but the cannot find a unified set of formula for UC3842. Help!
Attached is the schematic of one of my two UC3842 units. This is the fixed auxiliary rail regulator, outputting +/- 12V for analog section and +5V for digital section. The input voltage (at VIN net) is 400V, from a UCC28019-based active PFC circuit.
Questions:
1) How to calculate the components?
2) How to make this regulator adjustable without throwing power ratings out of the window? The primary regulator have an output range of 2.5-32.5V at 0-10A, with output power 50W maximum.
T3sl4co1l:
R27, C16: Oscillator frequency. Arbitrary. Pick a frequency suitable for the project. Typically 100-130kHz.
R26, R28, R32, C18: Opto feedback path. C18 = 0, resistors designed for proper voltage range. The topology is wrong. Normally, you wire the internal error amplifier as gain = -1, which would be done with resistors like so:
Which leads to the next parts,
D15: omit (replace with short), add resistor across opto (pins 1-2), add R+C across IC4 ("cathode" to "adj"). This provides compensation. The supply is hopeless without the RC.
There's no aux winding. Will R29 be able to supply operating current? Normally, controller VCC is sourced from an additional winding on the transformer. This also removes any need for limiting supply or output voltage (D9-D11 can be removed completely), and significantly improves efficiency (R29 can be about 100k 1W).
R30: the UC3842 output stage can supply about 1A, so from VCC around 15V, the resistance should be about 15V/1A = 15 ohms. It can be smaller to improve speed slightly, or larger to reduce EMI.
R34 = 1V / Ipk
Ipk is determined by operating frequency, supply voltage, and output power. If Vin = 300V and P = 50W, then Iin = 0.167A DC. At 50% duty cycle, Iin = 0.333A average during the on-time. The on-time is linearly rising, so Ipk is twice this, or 0.667A. Add 20-40% more for tolerances, and you get maybe 0.8A, or 1.2 ohms.
R33 = 10k or optional.
R31, C17: typically 1k and 220pF. Time constant determined by transformer's stray capacitance and transistor's gate capacitance.
Q6 drain: needs a snubber. dV/dt preferred. C = Ipk / (2*Vin / tr). tr is the desired rise time, usually similar to the gate time constant (for IRF840, Cg ~= 6nF, and if R30 = 10 ohms, then tau = 60ns). 100pF would be typical here. The diode can be UF4007, and the resistor should give an RC time constant shorter than the minimum on-time. 10kohms would be fine.
The RC also serves as damping for the unloaded ringdown of the transformer. The RC values can also be chosen based on best damping there.
The transformer should have a primary inductance of L = Vin / (2*Ipk*Fosc), or about 2.2mH for this case.
The unloaded ringdown will be around 200pF (from Q6 Coss and reflected equivalent D12-D14 Cjo) and 2.2mH or 240kHz, which is pretty low, maybe low enough not to care. The impedance is sqrt(2.2mH / 200pF) = 3.3kohm, so the snubber R being near this value will give better damping.
A smaller transistor could also be chosen, especially a newer one with significantly reduced Ciss and Coss.
Lastly, output side: don't split grounds. You're inviting ground loop and worse EMI problems than whatever you imagine you're trying to avoid in the first place. Keep them all on common ground, and filter that ground.
With common ground, you can also jointly regulate supplies. Change R35 to 20k, and add an 80k resistor from IC4 "ADJ" to C23 (+12V, but before the LC filter). As shown, only the 5V supply is regulated, which means the 12V supply voltage will go crazy if the loads are mismatched.
Also, the LC filtering is not without scrutiny. You are making a C-L-C resonant circuit, which needs to be heavily damped, otherwise the supplies will likely be worse (due to ringing) than the noise you filter from them. The critical parameters are again R, L and C. The two capacitors (C23 and C25; C24 and C26) act in series, so have half the capacitance and twice the ESR of a single capacitor (assuming identical components). You might have 220uF and 1 ohm in this case; if the inductor itself also has an ohm or more DCR, then sqrt(220uH / 235uF) will be smaller than the total ESR+DCR, and the network will be overdamped (good).
Using less C, more L, or higher quality L and/or C (e.g., very low ESR aluminum polymer) would lead to problems. The values as shown, assuming reasonable component choice, are probably good.
Tim
technix:
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R27, C16: Oscillator frequency. Arbitrary. Pick a frequency suitable for the project. Typically 100-130kHz.
--- End quote ---
How good is 450kHz?
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R26, R28, R32, C18: Opto feedback path. C18 = 0, resistors designed for proper voltage range. The topology is wrong. Normally, you wire the internal error amplifier as gain = -1, which would be done with resistors like so:
Which leads to the next parts,
D15: omit (replace with short), add resistor across opto (pins 1-2), add R+C across IC4 ("cathode" to "adj"). This provides compensation. The supply is hopeless without the RC.
--- End quote ---
Thanks for the info here.
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---There's no aux winding. Will R29 be able to supply operating current? Normally, controller VCC is sourced from an additional winding on the transformer. This also removes any need for limiting supply or output voltage (D9-D11 can be removed completely), and significantly improves efficiency (R29 can be about 100k 1W).
--- End quote ---
I want to live without the aux winding. One less winding to calculate...
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R30: the UC3842 output stage can supply about 1A, so from VCC around 15V, the resistance should be about 15V/1A = 15 ohms. It can be smaller to improve speed slightly, or larger to reduce EMI.
--- End quote ---
Thanks.
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R34 = 1V / Ipk
Ipk is determined by operating frequency, supply voltage, and output power. If Vin = 300V and P = 50W, then Iin = 0.167A DC. At 50% duty cycle, Iin = 0.333A average during the on-time. The on-time is linearly rising, so Ipk is twice this, or 0.667A. Add 20-40% more for tolerances, and you get maybe 0.8A, or 1.2 ohms.
--- End quote ---
Great. Can I use 2512 chip resistors here? Also Vin = 400V not 300V, as I have an active PFC.
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R33 = 10k or optional.
R31, C17: typically 1k and 220pF. Time constant determined by transformer's stray capacitance and transistor's gate capacitance.
Q6 drain: needs a snubber. dV/dt preferred. C = Ipk / (2*Vin / tr). tr is the desired rise time, usually similar to the gate time constant (for IRF840, Cg ~= 6nF, and if R30 = 10 ohms, then tau = 60ns). 100pF would be typical here. The diode can be UF4007, and the resistor should give an RC time constant shorter than the minimum on-time. 10kohms would be fine.
The RC also serves as damping for the unloaded ringdown of the transformer. The RC values can also be chosen based on best damping there.
--- End quote ---
Can I whack an antiparallel diode there (1N5408 maybe) instead of a snubber? Just like buck converters?
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---The transformer should have a primary inductance of L = Vin / (2*Ipk*Fosc), or about 2.2mH for this case.
The unloaded ringdown will be around 200pF (from Q6 Coss and reflected equivalent D12-D14 Cjo) and 2.2mH or 240kHz, which is pretty low, maybe low enough not to care. The impedance is sqrt(2.2mH / 200pF) = 3.3kohm, so the snubber R being near this value will give better damping.
A smaller transistor could also be chosen, especially a newer one with significantly reduced Ciss and Coss.
--- End quote ---
Smaller transistor = $$$. IRF840, being an older but common design, is really, really cheap. Also since this transistor is also used on the main converter as well as the PFC converter, it is one component less to concern for me.
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---Lastly, output side: don't split grounds. You're inviting ground loop and worse EMI problems than whatever you imagine you're trying to avoid in the first place. Keep them all on common ground, and filter that ground.
With common ground, you can also jointly regulate supplies. Change R35 to 20k, and add an 80k resistor from IC4 "ADJ" to C23 (+12V, but before the LC filter). As shown, only the 5V supply is regulated, which means the 12V supply voltage will go crazy if the loads are mismatched.
--- End quote ---
Then how do I do it with three voltages, each with drastically different current ratings?
+5V rail needs to be set at 5.2-5.3V and capable of a maximum output of 3A (as it may be used to charge an iPad and power an Raspberry Pi at the same time, which can pull up to 2A and 1A respectively)
+12V drives op amps and a fan, requires a maximum of 1A (fan + op amps)
-12V is only used by op amps so up to 200mA is required.
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---Also, the LC filtering is not without scrutiny. You are making a C-L-C resonant circuit, which needs to be heavily damped, otherwise the supplies will likely be worse (due to ringing) than the noise you filter from them. The critical parameters are again R, L and C. The two capacitors (C23 and C25; C24 and C26) act in series, so have half the capacitance and twice the ESR of a single capacitor (assuming identical components). You might have 220uF and 1 ohm in this case; if the inductor itself also has an ohm or more DCR, then sqrt(220uH / 235uF) will be smaller than the total ESR+DCR, and the network will be overdamped (good).
Using less C, more L, or higher quality L and/or C (e.g., very low ESR aluminum polymer) would lead to problems. The values as shown, assuming reasonable component choice, are probably good.
Tim
--- End quote ---
About that LC filter for analog ground, if I use 680uH L's and 47uF tantalums, will it be better than the 470uH's and 220uF aluminum I have here?
T3sl4co1l:
--- Quote from: technix on May 01, 2016, 04:35:20 pm ---
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R27, C16: Oscillator frequency. Arbitrary. Pick a frequency suitable for the project. Typically 100-130kHz.
--- End quote ---
How good is 450kHz?
--- End quote ---
I wouldn't recommend it, especially not with IRF840, and especially not if you're being shy with your transformer design.
--- Quote ---I want to live without the aux winding. One less winding to calculate...
--- End quote ---
Calculate what? It's just a ratio. In fact, it will be fine to use exactly the number of turns as your +/-12V windings!
--- Quote ---Great. Can I use 2512 chip resistors here? Also Vin = 400V not 300V, as I have an active PFC.
--- End quote ---
I gave the formulas, so you can substitute the different numbers and get the correct results.
How much power is a 2512 rated for?
The power dissipation will be up to P = R * Ipk^2 / 6. ((Neat calculus exercise for the student: show why this is true.)
--- Quote ---Can I whack an antiparallel diode there (1N5408 maybe) instead of a snubber? Just like buck converters?
--- End quote ---
I can think of about four things wrong with that suggestion, but the most important one should be obvious from looking at the circuit...
Hint: what does primary voltage necessarily rise to, when the transistor turns off?
--- Quote ---Then how do I do it with three voltages, each with drastically different current ratings?
+5V rail needs to be set at 5.2-5.3V and capable of a maximum output of 3A (as it may be used to charge an iPad and power an Raspberry Pi at the same time, which can pull up to 2A and 1A respectively)
+12V drives op amps and a fan, requires a maximum of 1A (fan + op amps)
-12V is only used by op amps so up to 200mA is required.
--- End quote ---
A widely variable load is probably not a good idea, for this type of power supply, and it supposedly being a "auxiliary" supply.
Joint regulation gets you the best compromise, at least among the supplies being regulated.
You can't mix the -12V in, without using an inverting feedback stage, so you'll just have to ignore it. (Or use a much more complicated feedback circuit, but I doubt it's worth it.)
Probably none of the supplies need to be very accurate, and as long as there is a modest idle load, the worst fluctuations won't be dangerous.
Op-amps don't much care about their supply voltages, as long as they remain within ratings. If you do require precise -12V and other supplies, you will need to consider a different topology.
--- Quote ---About that LC filter for analog ground, if I use 680uH L's and 47uF tantalums, will it be better than the 470uH's and 220uF aluminum I have here?
--- End quote ---
I gave the formula and condition, so you can substitute for those values and see whether it is a good idea or not.
I would guess no.
You also didn't specify what is "better", say in terms of dB attenuation at whatever frequency. Since you've given no noise or filtering spec, no reason to include themm I would just as well remove them, leaving the power raw.
Tim
technix:
--- Quote from: T3sl4co1l on May 01, 2016, 05:29:47 pm ---
--- Quote from: technix on May 01, 2016, 04:35:20 pm ---
--- Quote from: T3sl4co1l on May 01, 2016, 04:07:35 pm ---R27, C16: Oscillator frequency. Arbitrary. Pick a frequency suitable for the project. Typically 100-130kHz.
--- End quote ---
How good is 450kHz?
--- End quote ---
I wouldn't recommend it, especially not with IRF840, and especially not if you're being shy with your transformer design.
--- End quote ---
So since 100kHz is a good idea I will stick to that, slightly higher maybe? (125-130kHz)
--- Quote from: T3sl4co1l on May 01, 2016, 05:29:47 pm ---
--- Quote ---I want to live without the aux winding. One less winding to calculate...
--- End quote ---
Calculate what? It's just a ratio. In fact, it will be fine to use exactly the number of turns as your +/-12V windings!
--- End quote ---
UC3842 requires at least 16V to start up but the transistor have a maximum Vgs of 20V. Goldilocks much?
--- Quote from: T3sl4co1l on May 01, 2016, 05:29:47 pm ---
--- Quote ---Great. Can I use 2512 chip resistors here? Also Vin = 400V not 300V, as I have an active PFC.
--- End quote ---
I gave the formulas, so you can substitute the different numbers and get the correct results.
How much power is a 2512 rated for?
The power dissipation will be up to P = R * Ipk^2 / 6. ((Neat calculus exercise for the student: show why this is true.)
--- End quote ---
2512 size is rated for 1 watt if given proper thermal design (and since I will tuck all three current sense resistors, two for UC3842 and one for PFC, under the mains side heatsink I think I have adequate thermals for it)
--- Quote from: T3sl4co1l on May 01, 2016, 05:29:47 pm ---
--- Quote ---Can I whack an antiparallel diode there (1N5408 maybe) instead of a snubber? Just like buck converters?
--- End quote ---
I can think of about four things wrong with that suggestion, but the most important one should be obvious from looking at the circuit...
Hint: what does primary voltage necessarily rise to, when the transistor turns off?
--- Quote ---Then how do I do it with three voltages, each with drastically different current ratings?
+5V rail needs to be set at 5.2-5.3V and capable of a maximum output of 3A (as it may be used to charge an iPad and power an Raspberry Pi at the same time, which can pull up to 2A and 1A respectively)
+12V drives op amps and a fan, requires a maximum of 1A (fan + op amps)
-12V is only used by op amps so up to 200mA is required.
--- End quote ---
A widely variable load is probably not a good idea, for this type of power supply, and it supposedly being a "auxiliary" supply.
Joint regulation gets you the best compromise, at least among the supplies being regulated.
You can't mix the -12V in, without using an inverting feedback stage, so you'll just have to ignore it. (Or use a much more complicated feedback circuit, but I doubt it's worth it.)
Probably none of the supplies need to be very accurate, and as long as there is a modest idle load, the worst fluctuations won't be dangerous.
Op-amps don't much care about their supply voltages, as long as they remain within ratings. If you do require precise -12V and other supplies, you will need to consider a different topology.
--- End quote ---
The +5V rail always have a MCU and a BLE module on there, so it always have a few milliamps. However the load can vary all the way up to a few amps. My main op amp is TL084, can I live without a regulated power rail for that?
Also if UC3842 don't like wildly varying outputs, how to construct an adjustable mixed-mode bench power supply using it as the primary regulator?
--- Quote from: T3sl4co1l on May 01, 2016, 05:29:47 pm ---
--- Quote ---About that LC filter for analog ground, if I use 680uH L's and 47uF tantalums, will it be better than the 470uH's and 220uF aluminum I have here?
--- End quote ---
I gave the formula and condition, so you can substitute for those values and see whether it is a good idea or not.
I would guess no.
You also didn't specify what is "better", say in terms of dB attenuation at whatever frequency. Since you've given no noise or filtering spec, no reason to include themm I would just as well remove them, leaving the power raw.
Tim
--- End quote ---
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