Talk:Forward converter

Various comments

I have added an illustration of the difference between the forward converter and the flyback converter as the original ( unaltered ) very short entry mentioned the flyback converter as similar in function to the forward converter. I thought that a comparison between the two would be enlightening as the essential difference is that the flyback stores energy in an inductor and transfers it to the output during the non-conduction phase of the power switch whereas the forward converter does not store energy, relying on transformer action and transferring energy 'forward' to the output during the power swicth conduction cycle. Needs graphics and tidying really.94.197.177.20 (talk) 00:26, 22 June 2010 (UTC)

This article may be confusing or misleading... The term 'flyback converter' is commonly used to describe transformer-based converters; the flyback transfers energy to the output during the off time of the primary switch. Transformers are equivalent to coupled inductors; they can store energy in the same way as inductors. —Preceding unsigned comment added by 110.174.229.46 (talk) 08:12, 7 November 2010 (UTC)

A flyback converter can make use of an inductor and not a transformer -- this is quite common for step-up conversion. In fact if a transformer is used in 'flyback mode' it needs to be considered (in the design sense carefully as) both inductor and transformer (i.e. an inductor with an auxiliary winding on the same core, isolated or not); the magnetizing inductance becomes an important part of the design. The chief distinction between a flyback converter and a forward converter is the winding-sense of the secondary + the fact that the "drive-sw8tcy" is (usually) a single transistor or a single pair both turned on at the same time for less than 50% duty cycle (but all these need caveats in high-power design (i.e. many kilowatts), many of these topologies are very sophisticated). The advantage of single-switch-forward versions is that there's little possibility of flux imbalance; however the power delivered is less than that delivered by symmetric switching topologies that use the "whole core" (but these need duty-cycle control with a dead time to prevent flux imbalance; or, a capacitor-coupled load, etc.).

This article as written is quite accurate and describes things correctly; the first designs (historically) had a "flux reset" secondary winding (plus a diode) as well as the tertiary (load) windings (Once I had the original reference, it's an oldie about 1978; I believe it is to be found in Electronics Magazine, an article by a researcher from Philips; nope, my cc is gone, I looked...). Even a forward converter can be wound as a coupled inductor; again the fact that (i) there's (almost always) a single switch, and (ii) a transformer-like topology (transformer or coupled inductor, usually a closely-coupled transformer), and (iii) the winding sense is chosen so that the load winding(s) conduct(s) at the same time that the switch is closed and the primary is conducting (the flux reset winding conducts during the off time of the switch), is what matters. Bill Wvbailey (talk) 01:22, 9 December 2010 (UTC)

No. The article doesn't provide any kind of explanation of the difference except that suddenly the same construct is magically considered an inductor instead of a transformer if you use a different word. There is no causality in the explanation. The schematics in the respective articles are also completely different and very elaborate showing much more than the minimal or important parts, let alone differences.

Here's a brilliantly clear and simple diagram. Is this the difference, a *second* independent inductance? http://ars.els-cdn.com/content/image/1-s2.0-S0196890408000447-gr16.jpg 91.154.87.201 (talk) 10:58, 21 November 2012 (UTC)

You ask an interesting question, how to turn a forward converter topology into an inductor-only (no-transformer) "chopper" topology (that's what we called them back in my day, i.e. no transformer for galvanic isolation). I need to ponder this a bit: Let's see . . . if you put the switch above the inductor and connect a "freewheeling diode" from ground up to the bottom of the switch and to the switch end of the inductor, this is the "forward" mode; when the switch closes it delivers voltage across and current to the inductor; then when the switch opens the current in the inductor and load "free wheel" through the diode. The "flyback" way to do this is swap the positions of inductor and diode, this can deliver very high voltages. A variant to this puts the inductor "above" the switch. This is the typical "boost" (really flyback) topology; the forward version is a bit harder to figure out because it not so common as the "boost" version.

As to your question if I understand it: When a transformer is present, No, the secondary inductor isn't the significant difference, it's just used to "filter" the "chopped" secondary voltage and turn it into DC voltage (that appears across the inductor). The difference between forward "mode" and flyback "mode" is (1) the tight coupling (usually) of the transformer and (2) the winding sense and (3) presence and direction of the first diode in the secondary that constrains secondary current to flow in the "forward mode" i.e. when the switch closes. What you will see at the secondary is "chopped DC voltage" (and current, similar to the explanation above), and that is what the filter-inductor and its freewheeling diode are turning into DC current with some ripple on it. Usually forward converters are designed with very tight coupling to reduce the leakage inductance, but not always. See the explanation under the photo in the section below -- energy is being transferred to the diode in both "modes" -- forward and flyback (the resulting DC is quite "ripply", and there's no output inductor; to get the voltage- boost to light the LED in forward mode the transformer is required to be "step up", then in flyback mode the leakage inductance increases the flyback voltage enough to exceed the LED's voltage). BillWvbailey (talk) 16:34, 21 November 2012 (UTC)

Drawings of a forward converter (in the classical sense)

I've created two drawings.

The top schematic identifies the parts of the circuit. The bottom schematic shows the directions of voltages and currents used in the analysis.

The waveforms associated with the schematics. See the text for definitions of the various abbreviations. The primary current is rising at the rate V_bat*t/L_p, where t is time from the instant switch SW1 closes, L_p is the open-circuit primary inductance, otherwise known as the magnetizing inductance.

An assembled blocking oscillator that works in both forward and flyback modes. The bottom image shows the transformer with the top-half of the core removed; the LED still lights, but at a reduced intensity. The two schottky diodes in the upper left are responsible for directing current to the 0.47 microfarad filter-capacitor and the LED. The left-most diode is connected to the transformer's secondary winding and conducts in the forward mode i.e. when the switch is closed. The right-most diode is connected to the transformer's primary, and it directs the magnetizing current to the LED/capacitor when the switch opens, i.e. during "flyback" (similar to diode D in the schematic, excepting it is directing current to the LED rather than back to the battery). The "switch" is an NPN transistor operating with another NPN in a current-mirror configuration -- the two two are glued together. Below this pair is another transistor that turns the oscillator on and off depending on the ambient light; on the far right is the cadmium-sulphide light-sensor (covered with blue tape so the circuit will illuminate the LED).

Of course, these need to be verified for accuracy. Input is welcome! The rising currentWvbailey (talk) 22:10, 17 January 2011 (UTC)Bill

SW1: SWitch #1

SW2: SWitch #2

V_B:voltage across the battery (source)

V_P:voltage across the primary winding

V_S:voltage across the secondary winding

V_D:voltage across the diode (e.g. flyback voltage will cause forward-conduction through diode)

V_sw1:voltage across the primary-circuit switch #1

N =_def turns ratio i.e. N = N_S/N_P = Number_{turns, Secondary}/Number_{turns, Primary}

e.g. If primary has 20 turns and the secondary has 60 turns then the turns ratio "1:N" is 1:3.

I_P: current through the primary winding. During the forward-conduction time this is the same as I_sw1 and I_battery. During the flyback time this is the same as I_diode

I_D: I_diode, diode current

More to follow . . . Bill Wvbailey (talk) 18:52, 19 January 2011 (UTC)

Since we are talking about a forward converter here, in order not to cause more confusion, I suggest we use the term core reset rather than flyback? Also since you to explain things very well Bill, we might want to add that the reset voltage can be problematic with a step-up transformer as I and Linear Technology have recently found out. If I remember correctly we are talking Vin*1/1-Duty Cycle on the primary and times the turns ratio on the secodary. In my case of 12 volt input and 1:3 transformer and 75% Duty Cycle you have a reset voltage on the secondary of 144 volts. --  :- ) Don 09:55, 5 September 2012 (UTC)

Voltage ratio

I assume V_out/V_supply is a DC ratio. However, I really don't see that. Is V_supply U_E, V_p or V_s?

ICE77 (talk) 07:25, 9 January 2012 (UTC)

See my drawings above. The battery voltage Vb is of course d.c. but Vs is the amplitude of the secondary-winding's pulse; the primary voltage Vp consists of two pulses: Vb in the forward direction and Vflyback_diode (e.g. Vzener) shown as a negative pulse. If it were rectified, filtered with a capacitor only, and lightly loaded, then Vout would be d.c. and the ratio would be Vout = Vs/Vb. If heavily-loaded, Vout would droop (computation-- difficult). But if after rectification (and now you need an extra flyback diode) Vs were filtered with an inductor first, Vout would have to include the duty cycle d: Vout = d*Vs/Vb. If you're clever about it (e.g. add another winding to form a blocking oscillator, or use the secondary winding for the purpose) and you don't care about transformer isolation, you can dump the primary winding's flyback energy (via the flyback diode) into the load too: total of 2 shottky diodes. That's what the little circuit lighting the LED is doing (thus it's a blocking oscillator, a flyback and a foward converter. Altho it's probably O.R. I consider it in the public domain). Bill Wvbailey (talk) 16:11, 9 January 2012 (UTC)

Still, that doesn't answer my question. Assuming that the converter is a 20V-to-5V converter, is Vout/Vsupply the ratio 20V/5V? That's all I need to know.

ICE77 (talk) 02:30, 27 February 2012 (UTC)

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Page/source discrepancy

Page has the following near the end:

The forward converter is typically used in off-line supplies to provide an intermediate power output level of 100–200 watts.[2]

The [2] source, Power Electronics by Daniel Hart (ISBN 978-0-07-338067-4), in fact says this (page 299, section 7.11):

The forward converter is a popular circuit for low and medium power levels, up to about 500 W. It has one transistor as does the flyback, but it requires a smaller transformer core. Disadvantages are high voltage stress for the transistor and the extra cost of the filter inductor. The double-ended forward converter can be used to reduce the switch voltage stress, but the drive circuit for one of the transistors must be floating with respect to ground.

There is no mention in the source of the 100-200 W figure provided on the wiki page. However, the other reference in the article, "Single Transistor Forward Converter Design," states:

The single transistor forward converter is commonly used for off-line supplies in the power range below 200W.

(That citation also contains a dead link---I had to access at https://www.mouser.com/pdfdocs/2-10.pdf, maybe use that, save on archive.org)

Other sources I find (presentations, magazine articles, publications, similar power engineering textbooks) put the forward converter topology in the "mid-power" (100-500 W) range, though with different sub-topologies. From p. 181 of Switching Power Supplies A to Z by Sanjaya Maniktala, 2nd edition (ISBN 978-0-12-386533-5):

Just as the isolated flyback is a derivative of the Buck-Boost topology, the Forward converter is the isolated version (or derivative) of the Buck topology. It too uses a transformer (and optocoupler) for providing the required isolation in high-voltage applications. Whereas the flyback is typically suited for output powers of about 75 W or less, the Forward converter can go much higher. The simplest version of the Forward converter uses only one transistor (switch), and is thus often called “single-ended.” But there are variants of the single-ended Forward converter with either two or four switches. So, although the simple Forward converter is suited only up to about 300 W of power, we can use the “double-switch Forward converter” to get up to about 500 W. Thereafter, the half-bridge, push-pull, and full-bridge topologies can be exploited for even higher powers (see Figure 4.2 and Table 7.1). But note that all of the above topologies are essentially “Buck-derived” topologies.

If anything, the article should at least be updated for the content-reference discrepancy. 2601:248:527F:AF40:CB7E:19B6:790A:89D2 (talk) 16:31, 11 January 2024 (UTC)