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  1. #1
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    [moved] Switch mode power supply interview question and response...bluffing?

    At an interview today, the engineer told me that Voltage mode controlled SMPS’s need slope compensation to be added when the duty cycle goes >0.5………he said that this slope compensation would take the form of increasing the slope of the already_present synthesized ramp that exists in voltage mode converters….he drew it for me, and his synthesized_ramp was piecewise linear …ie a straight line with slope A, which then “kinked” to a straight line of slope B (>A). So his synthesized ramp was like “dog-legged” kind of thing.

    I actually suspect he was bluffing me, would you agree?

    He also told me that Voltage mode converters were simpler than current mode converters.
    We were talking with reference to a Buck converter, and I pointed out the power stage double pole that exists with voltage mode control, but this left him not impressed.
    He also told me how a current sense transformer in a Buck converter would best go downstream of the inductor. He pointed out that where I had placed a CST (upstream of the FET) was an unwise choice.

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  2. #2
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    Re: [moved] Switch mode power supply interview question and response...bluffing?

    I actually suspect he was bluffing me, would you agree?
    No good interview concept, I rather think, he was out of his depth.

    He also told me how a current sense transformer in a Buck converter would best go downstream of the inductor. He pointed out that where I had placed a CST (upstream of the FET) was an unwise choice.
    Both are reasonable choices, depending on the purpose. Was it mentioned at all?


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  3. #3
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    Re: [moved] Switch mode power supply interview question and response...bluffing?

    No slope comp needed in volt mode converter - but you do use the Vin to modulate the ramp to the same effect ( iron out 100Hz ripple on Vin)

    slope comp needed on current mode - which is slightly more complex, you can add slope comp only above 45% duty cycle, hence a sort of dog leg - but not for volt mode - I think he had these two confused

    CST is best on o/p side of buck choke as the volts are fixed there and you get less CM pickup than you would for the noisy switching side ...


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  4. #4
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    Re: [moved] Switch mode power supply interview question and response...bluffing?

    It could be that his strategy in conducting an interview, is to find out whether he'll get the most valuable input you can give to future projects in the company. Thus he might present an idea which goes against normal practice (and he knows it and realizes most job applicants should know it too). Your best cautious answer is probably to state what is the normal wisdom in the situations he brought up, but then admit there are circumstances which require tailoring the design so it works as it's supposed to.

    He might watch for warning flags:

    * Automatic agreement (insufficient knowledge, no confidence in your own opinion, 'yes' man)

    * Irate response (inflexibly rooted in convention, difficult to work beside, never considers ideas from coworkers or even superiors).


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