WITH ST's ADVANCED CONTROLLERS Claudio Adragna Giuseppe Gattavari

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WITH ST's ADVANCED CONTROLLERS Claudio Adragna Giuseppe Gattavari

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HANDLE SHORT CIRCUIT CONDITIONS
WITH ST's ADVANCED CONTROLLERS
Claudio Adragna Giuseppe Gattavari
Purpose this note provide some advice manage short circuit condition switching converter controlled ST's advanced controllers L5991 L5993. After quick review merits limits current limitation functionality embedded these IC's, simple circuit will given that allows fulfill designer's typical requirements what converter supposed case overload short circuit.
Current limitation schemes: merits limits ST's advanced controllers L5991 L5993 include two-level overcurrent protection: "pulseby-pulse" "cycle-by-cycle") current limitation so-called "hiccup mode" operation. Pulse-by-pulse current limitation ideally limits peak primary current 1/Rs (see figure when converter overloaded there load short circuit. Even neglecting non-idealities, which worsen scenario must accounted safe design, pulse-by-pulse current limitation just prevents system from losing control peak primary current. Nothing more. Figure control current limiting L5991 L5993.
Vout
ERROR AMPLIFIER
CLOCK COMPARATOR LATCH
DRIVER
Vdrain
ADVANCED CONTROLLER (L5991, L5993)
HICCUP 1.2V SOFT-START
ISOLATED FEEDBACK
December 1999
output current limited cause traces burning. Converter's power throughput declines only slightly. Furthermore, power losses concentrated some specific parts, e.g. output rectifier(s), which overheat towards destruction rated this extreme condition heatsinking capability generously provided. When pulse-by-pulse current limitation cannot maintain peak primary current under control, hiccup mode protection takes over halts resulting current runaway. This happens when switch ON-time needed control peak primary current lower than minimum value ONmin that controller generate result internal delays. L5991 L5993 TONmin estimated high typical value. More details this "runaway condition" found appendix. With reference again figure case current runaway voltage current sense will over reaches 1.2V comparator triggers this special functionality [1], [2]. Unlike pulse-by-pulse limitation, hiccup mode operation keeps output current power throughput very low. would desirable, then, rely hiccup operation conveniently handle load short circuit. However, quite common that even dead short converter's output cannot activate hiccup protection. Referring appendix detailed analysis, high input voltage, high regulated output voltage high switching frequency favour current runaway and, therefore, hiccup operation tripped further load short circuit. many applications, such AC-DC adapters PC's silver boxes, where output voltage high switching frequency quite (below kHz) extremely likely that runaway condition will never hiccup protection will never activated. Furthermore, these applications load usually connected through long cables, then total short circuit resistance needed control peak current needs short. other applications, such monitor SMPS, runaway condition likely highest output voltages, especially high input voltage they synchronized high frequency. Actually, hiccup protection serves different purposes. first related capacity L5991 L5993 operated very high frequency beyond kHz): guarantee safe handling load short circuit under conditions operating frequency, hiccup protection must added pulse-bypulse limitation. second purpose cover some fault conditions that pulse-by-pulse limitation cannot handle. These basically: short circuit secondary winding (regardless topology); secondary catch diode short circuit flyback converters); secondary freewheeling diode short circuit forward converters).
Figure Fault conditions that activate hiccup mode operation.
Secondary winding short circuit (any topology)
Secondary diode short circuit (flyback topology)
Freewheeling diode short circuit (forward topology)
Equivalent circuit
these conditions reflect short circuit transformer's primary side while switch primary current rate rise then limited only leakage inductance transformer, which percent primary magnetizing inductance (see fig. This means that current slope will 30-50 times higher than normal operation, thus pulse-by-pulse limitation chance control current.
Overcoming pulse-by-pulse issues With reducing power consumption during overload load short circuit, possible select following basic strategies: Making converter enter hiccup mode operation fault detected letting back into normal operation automatically fault removed (autorestart). Shutting down converter fault detected keeping until disconnected from mains. Only after power-off power-on cycle converter enabled restart.
Both these strategies easily implemented with ST's advanced family controllers, thanks numerous functions available. Figure shows this done with same basic circuit comprising only four cheap external parts. Figure Different strategies dissipation during load short circuit
BC327
BC327
0.47
Vref COMP
Vref COMP 1N4148
1N4148
L5991 L5993
SGND
ISEN
L5991 L5993
SGND
Autorestart
Shutdown
When current limitation because overload load short circuit, output voltage drops below regulated value output error amplifier (pin COMP) saturates high attempt recovering voltage regulation. This used turning small-signal (its base tied getting current available triggering protection functionality. However, delay several needed prevent function from being activated start-up result load surge. This simply done with cell. Finally diode needed decouple circuit from others connected same pin, during normal operation. Hiccup activated pulling current sense (ISEN, above 1.2V. Error amplifier source capability with high saturation voltage about thus resistor between current sense resistor ISEN increased from collector resistor 10µF capacitor form delay cell about
This circuit been implemented 45W, wide range Figure Input power input voltage during hiccup mode operation mains AC-DC Adapter with L5991 results shown fig. Shutdown will activated injecting current delivered into (DIS) that voltage exceeds 2.5V. resistor divider R5-R6 refers circuit described part latched circuit. case different value collector resistor delay capacitor will changed consequently. Besides increasing safety, advantage each these techniques that heatsinking power parts needs designed neglecting anomalous operating conditions (overload short circuit), with considerable saving size cost.
L5991-based AC-DC Adapter Dead short output
Conclusions Pulse-by-pulse hiccup mode current limitation ways ST's advanced controllers L5991 L5993 handle overload short circuit conditions. been shown that these functions cover different fault categories: pulse-by-pulse does allow power throughput rise control case load failure whereas hiccup usually protects against short circuits inside converter itself (transformer, diodes) where pulse-by-pulse effective. Unlike hiccup, which dramatically reduces power throughput, pulse-by-pulse limitation little does about that, thus forcing designers oversize heatsinking critical parts sake safety. simple versatile circuit been presented that helps overcome this problem used achieve common protection schemes, autorestart shutdown, resulting very power dissipation. appendix, calculation method that helps evaluate whether load short circuit activate hiccup mode protection given application, provided.
REFERENCES "L5991/A Primary Controller with Standby" Datasheet "L5993 Constant Power Controller" Datasheet "45W AC-DC Adapter with Standby Function" (AN1134)
APPENDIX Will hiccup mode activated Basically, hiccup mode will activated control able guarantee steady-state operation converter under given short circuit condition. Like normal operation, under overload short circuit conditions steady-state operation requires balance across storage element magnetic energy: transformer flyback converter output inductor forward. Otherwise, current runaway will occur: energy rise inside magnetics during MOSFET's ON-time exceeds energy decay during OFF-time, thus current will raise with control saturating magnetics leading converter destruction. condition current runaway occur (runaway condition) will derived both flyback forward topology. Please refer appendix symbol explanation.
Flyback topology. balance condition (assuming system working continuous current conduction, which extremely likely under overload condition absolutely certain short circuit)
(A1).
Solving eqn. (A1) yields switch ON-time. This applicable during both normal operation pulseby-pulse current limitation: (A2).
worthwhile reminding that reflected voltage given (A3).
Whereas normal operation regulated and, therefore, constant, case overload short circuit drops does Their value will dependent total resistance secondary circuit, including diode, secondary winding wire, tracks, short circuit resistance. Obviously, more severe overload, lower total resistance, lower Eqn. shows that, overload gets closer short circuit, gets shorter shorter. time resulting from (A2) lower than TONmin, shortest switch ON-time that controller generate (200 typ. L5991 L5993), then controller will able maintain balance current will begin rise with control. Combining equations (A2) (A3), runaway condition, that condition hiccup operation invoked, (A4)
possible estimate left side eqn. assuming (ideal short circuit). real-world condition will greater than zero thus will this underestimated. Ideally, eqn. (A4) becomes: (A5)
this equation reasonable assume Schottky diode type. inspection (A5) shows that conditions that favour current runaway high input voltage Vin, high switching frequency (i.e. shorter high regulated output voltage: fact, being fixed MOSFET breakdown issues (typically around 100V), higher lower
Forward topology. balance condition (continuous current conduction even normal operation) output inductor
Solving eqn. (A6) yields switch ON-time: (A7). (A6)
Considering equation (A7), runaway condition forward converter Ideally, with this equation becomes: ONmi (A9) (A8)
where assumed again Schottky diode type. Like flyback topology, conditions that favour current runaway high input voltage, high switching frequency high regulated output voltage. However, comparing equations under same conditions Tsw, possible note that forward converter less prone than flyback current runaway. This reinforced fact, visible above equations, that forward converter output inductor resistance will always part short-circuit current loop.
Symbol list
Symbol TONmin Converter's input voltage. Converter's output voltage. Power switch ON-time. Minimum switch ON-time manageable controller flyback, secondary voltage reflected back primary during secondary rectifier conduction. Switching period, equal reciprocal converter's switching frequency. Transformer's primary-to-secondary turn ratio. Forward drop across secondary rectifiers (catch freewheeling diode) Description
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WITH ST's ADVANCED CONTROLLERS Claudio Adragna Giuseppe Gattavari

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