Low quiescent current with asynchronous buck converters at high temperatures?
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mso-level-text:; mso-level-tab-stop:108.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level4 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:144.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level5 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:180.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level6 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:216.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level7 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:252.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level8 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:288.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} @list l0:level9 {mso-level-number-format:bullet; mso-level-text:; mso-level-tab-stop:324.0pt; mso-level-number-position:left; text-indent:-18.0pt; mso-ansi-font-size:10.0pt; font-family:Wingdings;} ol {margin-bottom:0cm;} ul {margin-bottom:0cm;} –> This article discusses the contributors to no-load-input current in addition to the Iq of the converter itself, and the associated temperature dependency. Furthermore, we analyze when this effect is of concern and what to do about it.
Contributors to “no-load-input-current”:
The quiescent current (Iq) of a converter is one contributor only, and also its definition and test conditions indicate that in reality, the converter will show a higher current consumption at no load. Quiescent current of a converter refers to the supply current drawn at no load, non-switching by biasing the feedback pin at reference-voltage or slightly above and potentially at room-temperature only. Consequently, non-switching indicates that no switching losses are accounted for, even though a converter will switch once in a while at no load. There will be some leakage and also a recharge of the boot-capacitors or a charge-pump is likely required, adding to the losses. A drift of the quiescent current over temperature may or may not be specified separately.
Luckily, the temperature drift of most converters is expected in the order of approximately 10% increase at higher temperatures. For test-purposes to specify the Iq of a converter, the feedback is biased to the reference voltage. Thus, the feedback-divider to sense the output voltage is eliminated. Consequently, the current flowing through it (mostly in the order of 10uA..50uA) as well as switching of the FETs is prevented and its associated losses saved. In reality, those effects do contribute to the “no-load-input-current”. The graphic illustrates the contributors that, in addition to the quiescent current, will draw current from the supply.
A step-down converter with a specified quiescent current of 30 uA will – at room-temperature – most likely draw in excess of 50 uA of no load-current, accounting for feedback-divider-current and switching losses.
However, this does not explain the orders of magnitude of increase with higher temperature. This is related to asynchronous converters only: the catch-diode plays a significant role in this case: the majority of diodes show an increased reverse current, most being about 100 times higher at hot temperature (85°C or higher) compared to room-temperature.
For low voltage/low current diodes, e.g. 10V, 500mA, one can find diodes with as low as 1uA rated reverse current at room temperature, which consequently rises to about 100uA at hot. And unfortunately, at low load, blocking will be the use-case for the vast majority of the time. For higher voltage/higher current diodes, say 40V, 4A, the few microamperes or few dozen microamperes at room-temperatures will account for several milliamperes at elevated temperatures.
Another factor to consider is the increase in reverse current with increased reverse voltage, here a rise of about one decade has to be expected from low voltages to maximum rated voltages. However, clearly defined diode datasheets usually specify the maximum reverse current at maximum rated blocking voltage (and at discrete temperatures). Consequently, any asynchronous converter will exhibit a higher no-load-current at higher temperature, heavily dominated by the catch-diode and its increase of reverse current with temperature.
When is this of concern?
Obviously, if the respective system is not exposed to relatively high temperatures, this is of no concern. Similarly, if the system is only operating in normal mode, hence normal switching operation as opposed to power-save or no-load mode, at higher temperature, this may be negligible. An example could be an automotive application, where a unit such as the front-view-camera, is either off when hot, like for a car parked in the sun, or in normal operation when the car is running and the alternator is working, and therefore a small loss in efficiency can be tolerated. Similarly in industrial applications, a system is hardly ever in “standby” but either off or in normal mode. For consumer systems that are battery driven, it could be applicable, depending on the expected temperatures. To which extent the effect is seen also depends on the blocking voltage, hence the battery voltage in most cases, and load-current which drives the diode selection.
What to do if this effect is of concern?
Keep cool! Admittedly, this is easier said than done. A more appropriate solution is to choose a synchronous converter which eliminates the diode and replaces it with an active FET. A slight increase still needs to be expected with temperature, but it is in the 10% ballpark with a synchronous converter, rather than orders of magnitude with the asynchronous converter and catch diode.
Conclusion
The quiescent current is only one contributor to the no-load-current consumption. For an asynchronous converter, the catch-diode’s reverse current is a significant contributor, in particular at high temperatures. In some applications, this may be irrelevant or tolerable. In case a low quiescent current at elevated temperatures is of concern, a synchronous converter is likely the better choice.
About the author:
Frank Dehmelt graduated from Fachhochschule München (Munich Polytech) as electrical engineer in 1997 and joined TI as a FAE for interface products for both, communication infrastructure and industrial. He extended his expertize into Dataconverters and Amplifiers when defining products in these domains for the European markets as a Systems engineer. In 2010 Frank changed his focus and joined the Mixed Signal Automotive Team as an Application engineer for Power-management, supporting both battery-connected products, as well as processor-supplies.
References:
• IQ: What it is, what it isn’t, and how to use it
• Efficiency of synchronous versus nonsynchronous buck converters
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