Switching losses ESR and transient response

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Discussion Overview

The discussion revolves around the impact of switching frequency, duty cycle, and switching losses on transient response in power supply systems, particularly in the context of motherboard power supplies and DC-DC converters. Participants explore the relationships between these factors, efficiency, temperature, and the characteristics of MOSFETs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants define switching frequency as the rate at which phases are switched on and duty cycle as the ratio of time an object is on to the total time it can be on, suggesting its relevance in PWM control schemes.
  • There is a claim that efficiency and temperature are significant limiting factors for transient response, although this is contested by some participants.
  • Switching losses from MOSFETs are discussed as a major factor that limits the maximum achievable switching frequency.
  • Higher ESR in bulk output capacitors is noted as a contributor to increased ripple, affecting transient response.
  • One participant suggests that the size of a MOSFET correlates with its resistance, implying larger MOSFETs may introduce more resistance.
  • Another participant questions the relationship between switching frequency and transient response, suggesting that increasing frequency could reduce overshoot and improve transient response.
  • Discussion includes the role of feedback loop speed, compensation characteristics, and DC-DC topology in determining transient response, indicating that these factors interact with switching frequency.
  • A later reply emphasizes that higher switching frequencies can reduce output ripple and improve transient response, but also highlights the trade-off of increased switching losses.

Areas of Agreement / Disagreement

Participants express differing views on the primary factors limiting transient response, with some emphasizing efficiency and temperature, while others introduce additional considerations such as feedback loop dynamics and control strategies. The discussion remains unresolved regarding the exact relationships and trade-offs involved.

Contextual Notes

Participants acknowledge that transient response is influenced by multiple factors, including feedback loop characteristics and control strategies, which may not be fully explored or agreed upon in the discussion.

ssb2245
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I need someone to tell me if what I am saying is true or BS?


Switching Frequency: Is simply defined as the frequency (amount of times) the same operation occurs, in our case the amount of times per second each phase is switched on.
Duty Cycle: is thought of as the time the object is on divided by the total time the object can be on. It can be expressed as a percentage or ratio. For our purpose and explanation we can also think of it as Vo/Vi=duty cycle. Duty Cycle is used in the control scheme of many PWMs.

Efficiency and temperature are probably the biggest limiting factors behind transient response

every time the phase is working switching losses occur from the MOSFETs so you can’t just ramp up switching frequency

Switching losses (from MOSFETs) theoretically are one of the most significant factors that set the upper cap on switching frequency.

one huge factor that affects transient response as much or more than switching frequency is bulk output capacitor parasitics (ESR). Higher ESR from a capacitor ends up causing more ripple.

Another theory is that the actual size of a MSOFET designates a proportion to resistance, larger the MOSFET, the more resistance.

?
 
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ssb2245 said:
I need someone to tell me if what I am saying is true or BS?


Switching Frequency: Is simply defined as the frequency (amount of times) the same operation occurs, in our case the amount of times per second each phase is switched on.
Duty Cycle: is thought of as the time the object is on divided by the total time the object can be on. It can be expressed as a percentage or ratio. For our purpose and explanation we can also think of it as Vo/Vi=duty cycle. Duty Cycle is used in the control scheme of many PWMs.

Efficiency and temperature are probably the biggest limiting factors behind transient response

every time the phase is working switching losses occur from the MOSFETs so you can’t just ramp up switching frequency

Switching losses (from MOSFETs) theoretically are one of the most significant factors that set the upper cap on switching frequency.

one huge factor that affects transient response as much or more than switching frequency is bulk output capacitor parasitics (ESR). Higher ESR from a capacitor ends up causing more ripple.

Another theory is that the actual size of a MSOFET designates a proportion to resistance, larger the MOSFET, the more resistance.

?

Much of what you say is true, but I'm not so sure about this:

"Efficiency and temperature are probably the biggest limiting factors behind transient response"

What is the context of your question?
 
I am writing a review on a motherboard, and it has a "souped" up power supply for the cpu.

I thought that switching losses from the MOSFETs hurt efficiency a lot and that is why we don't have motherboard power supplies that switch at 3mhz per channel. Plus the way computers are now you can't have a power supply with high temperatures.

If not efficiency and then temperatures, then what limits the transient response?

maybe i should have said efficiency/temps limit switching frequency?
 

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ssb2245 said:
I am writing a review on a motherboard, and it has a "souped" up power supply for the cpu.

I thought that switching losses from the MOSFETs hurt efficiency a lot and that is why we don't have motherboard power supplies that switch at 3mhz per channel. Plus the way computers are now you can't have a power supply with high temperatures.

If not efficiency and then temperatures, then what limits the transient response?

maybe i should have said efficiency/temps limit switching frequency?

Switching losses do increase with frequency, so that part of your analysis is correct.

Transient response depends some on the switching frequency, but also on the speed of the feedback loop and compensation characteristics. It also depends on the DC-DC topology, and on things like duty cycle limiting and other things that you do in your control loop. There are tradeoffs between things like transient response time and overshoot, for example, and interactions with things like current limiting schemes.
 
ok its a mixed signal analog interleaved synchronous buck controller. So the feed back loop isn't controlled by PID or digital techniques. It uses current sensing across a voltage drop across a resister on every phase.

I thought if switching frequency was increase overshoot and undershoot are greatly reduced? so if you increased the SF then wouldn't you increase transient response and overshoot?

Some extra stuff if you don't mind looking at it:

"This is probably one of the most important issues any designer has to deal with, and this is where the PWM comes into play. High quality analogue and digital PWMs have excellent voltage regulation. Basically the way it works right now, is that once a user sets a voltage (we will call it Vref) the PWM then takes the proper steps to create this voltage. The voltage which is actually outputted is almost always different than what is set, so that voltage (we will cal Verror) is compared against the Vref and then the difference is used to change duty cycle and to power or turn off MOSFETs. The way that it goes from the difference between Vref-Verror to on/off of MOSFET is where the huge difference between PWMs come in. (Covered in Analogue VS. Digital the battle of the unequal equals) "
 
Hi everyone and i am sorry if i make this question to the wrong place but one old comment i just read intrigued my interest.

Could you please berkeman explain a little bit how switching frequency changes the transient respone? I can see about the feedback and the compensation characteristics but i have not a clear opinion about the switching frequency.

If possible a simple example to make it more clear!

Thanks!
 
_zeQ said:
Hi everyone and i am sorry if i make this question to the wrong place but one old comment i just read intrigued my interest.

Could you please berkeman explain a little bit how switching frequency changes the transient respone? I can see about the feedback and the compensation characteristics but i have not a clear opinion about the switching frequency.

If possible a simple example to make it more clear!

Thanks!

Welcome to the PF.

Higher switching frequencies in DC-DC converters mainly helps to reduce output ripple. It also can help move switching noise up above sensitive analog circuitry's passband in some applications. As for transient response, it can help improve it, especially with respect to overshoot on stepped load transients. Since there is finer granularity in the switching with a higher switching frequency, your control loop has "more chances" to respond to changes in output current loading.

The main disadvantage of using a higher switching frequency is increased losses in the switching transistor.
 

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