Sneaky peak: sneaky suggestions paths that de-stabilize an in any other case secure suggestions loop

I’ve lengthy held to the assumption that electrons are smarter than folks. Even one of the best engineers can fall prey to subtleties that electrons will readily act upon, particularly with regards to discovering sneak suggestions paths that may actually screw up an in any other case secure suggestions loop. We glance right here at a case examine.

There was this a number of output DC energy provide whose design was sometimes loop unstable and I used to be searching for the explanation why and searching for to discover a treatment. I arrange an injected sign, name that “E-test”, as proven in Determine 1 in order that by inspecting E2 with respect to E1, I may take a look at the achieve and part properties of the suggestions loop.

Determine 1 Fundamental loop achieve check plan for E-test.

There was a galvanic isolation barrier within the design, so the check setup was positioned as follows (Determine 2):

Determine 2 Loop achieve check plan in additional element.

We now take a look at how the Isolation Barrier Circuit was configured (Determine 3):

Determine 3 The alternating motion clamping isolation barrier circuit.

The sign enter voltage, E (on the left), will get transferred to the sign output voltage, E (on the best) by having a DC present supply that drives the transformer secondary heart faucet to induce alternate clamping motion through the 2 diodes on the transformer’s main. We lose some degree because of the Vcesat of the 2 NPN transistors and the ahead voltage drops of the 4 diodes, however a linear switch operate from enter to output may be very intently achieved. A extra detailed circuit is proven in Determine 4.

Determine 4 The choice motion clamping isolation barrier circuit in a bit extra element than Determine 3.

Please take psychological observe of the 8-volt energy provide on the 1N4623 zener diode. We are going to return to think about the character of these two elements just a little later.

This pair of curves proven in Determine 5 exhibits the output of the isolation barrier circuit and the following output of a  PWM management sign versus enter to the isolation barrier circuit. For the sake of suggestions loop management, that’s all we’d like.  

Determine 5 The isolation barrier circuit linearity.

Though now outdated, the Hewlett-Packard 4395A Community Analyzer was used for loop testing (Determine 6).

Determine 6 To the left, the HP 4395A community analyzer used for our E-test. To the best, its attachment to the unit beneath check (UUT).

The 4395A was connected to the UUT through the 1:1 interface transformer proven in Determine 7. The braid of the coaxial cable served because the check transformer’s main whereas the middle conductor of the cable served because the check transformer’s secondary. The 2 100 Ω resistors present an almost 50 Ω load for the analyzer’s RF output whereas the 100 Ω and 10 Ω resistors create a really small E-test to be able to maintain the ability provide’s working standing as near regular as doable whereas we do our measurements.

Determine 7 The check transformer and its attachment to the HP 4395A community analyzer.

We ran our loop achieve assessments at numerous ranges of excitation for E-test and acquired a giant shock.

 Because the check sign degree from the analyzer was taken from 0 dBm downward to -12 dBm, we had totally different check outcomes (Determine 8).

Determine 8 Loop achieve Seen for (a) 0 dBm, (b) -3 dBm, (c) -6 dBm, (d) -9 dBm, (e) -10 dBm and (f) -12 dBm excitation from the community analyzer.

Whereas the loop achieve roll-off attribute seemed good at first when the community analyzer was set to an output degree of 0 dBm, the roll-off attribute modified dramatically because the excitation degree was modified.

The wrongdoer was found as follows (Determine 9):

Determine 9 The sneak suggestions path.

The 8 volts of energy was being derived from the exact same inverter that the PWM motion was controlling, which led to a sneak suggestions path as proven above. The on resistance of the zener diode facilitated that path and, in my suspicion, Rzener assorted versus check excitation which led to the bizarre check outcomes.

The zener was changed with an lively IC as follows (Determine 10):

Determine 10 Eliminating the sneak suggestions path.

Through the use of the LM136 with its extraordinarily low dynamic resistance and altering one resistor to revive the PNP transistor’s Q-point, the sneak suggestions path was eradicated.

Take a look at outcomes grew to become the next (Determine 11):

Determine 11 Loop achieve and loop part with sneak path eliminated.

With the sneak suggestions path damaged, the gain-phase outcomes had been good and alike to one another in any respect ranges of check drive.

We had incorrectly assumed the ability provide was a linear system. Due to the zener’s habits, the ability provide was actually a non-linear system.

Ranging from scratch because it had been, loop achieve and loop part assessments ought to at all times be run at various ranges of excitation to see if the check outcomes match one another at each excitation degree. In the event that they don’t, you have got a non-linearity someplace that will trigger bother for you and/or in your finish person.

John Dunn is an electronics guide, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York College (MSEE).

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