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ISOM8110: Questions regarding photocoupler emulator ISOM8110

Akira Kurata
Intellectual 280 points
Part Number: ISOM8110
Other Parts Discussed in Thread: LM5023

 

Questions regarding photocoupler emulator ISOM8110

 

At the beginning of this data sheet, "switching power supply" is mentioned as an application for the photocoupler emulator.

In most switching power supply designs, dedicated control ICs are used in the power supply stabilization control section.

Also, in most cases, photocouplers are used to transfer input and output control signals.

The control signal between input and output that stabilizes this output voltage is often transferred using a current mode control signal using a constant current circuit.

Now, if we focus on this current input, most control ICs use a minute constant current signal of less than 500 μA.

It's under control.

Regarding the input/output characteristics of this photocoupler emulator evaluation board, which was prototype evaluated this time, the output current begins to flow when the input current is around 400 μA, and the range from this value to the upper limit is considered to be the practical control range.

Also, the data sheet seems to indicate that IF = 2 to 5 mA is the practical input current value range.

However, we recognize that when using a controller with most switching power supplies, the feedback control curren

t operates at the above 500μA or less.

Therefore, it is thought that the control current of this photocoupler emulator will operate at a current flow control current close to the lower operating limit.Wouldn't this control current value operation cause problems with the control characteristics due to the stabilization of the power supply?

When evaluating the actual implementation in an application, the current product (see Fig. 1) is stable in terms of gain (G) - phase (P) characteristics, but when used as a replacement for ISOM8110, the gain increases in the region higher than fT. There is an unstable part that appears to resurface (see Fig-2). 　

Isn't there any difficulty in stable control with the emulator control current value around 500μA?

Or do I need to increase the control current somehow?

Please give me some advice regarding the above.ISOM8110_G-P_20231102.pdf

  • 0
    TI__Mastermind 29595 points

    Hello Kurata-san, 

    Thank you for reaching out and the great information. 

    One of the intended use cases of ISOM8110 is isolated feedback signals for switching power supplies. As you have noticed, ISOM8110 has a lower IF limit of 500uA. The recommended IF region is 500uA - 20mA. Below 500uA the diode will be off. 

    If the compensation loop is tuned such that the operating region is 500uA and below, then it is necessary to increase the current compensation to be in the 500uA - 20mA range to ISOM8110. The benefit of ISOM8110 here is that the opto-emulator uses a non-light-based isolation technology which provides stable CTR over its lifetime and allows for a higher-bandwidth compensation loop. 

    Feel free to follow up if you have more questions. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 280 points

    SOM8110_Additional questions　　　　　　　　

    Thank you very much for your answer regarding the previous question.

    I understood from the answer that the lower limit of practical current for the photocoupler emulator_ISOM8110 is 500μA.
    Based on your response, the operating state of this emulator, which is used as an alternative coupler for an existing switching power supply that is currently being evaluated at our company, is close to this lower limit current. From the measurement results of the G-P characteristics, it is predicted that stable control cannot be achieved near the FT frequency in the high range of the control band.
    However, we have not received a response from your company regarding all of our questions.
    Once again, I would like to ask you to answer the following questions.
    <Re-question>
    (1) We recognize that the control current of the feedback signal input terminal of most switching power supply controllers currently on the market, including your company's PWM and LLC control ICs, is 500μ or less in the actual control state.
    Under these conditions, wouldn't it be problematic to replace the current ISOM8110 with an existing photocoupler replacement, as there is basically no operating margin in the control current range?
    Also, even if alternative operation is possible, stable performance may not be maintained due to the characteristics of the alternative circuit application for the reasons mentioned above.
    (2) If question (1) above is answered in the affirmative, this device is clearly intended for use as a switching power supply. Therefore, if it is an appropriate application, please introduce an application that uses this photocoupler emulator and allows the control current to be set in the stable recommended range (1 mA, etc.) far exceeding 500 μA.
    Please give us an example of a typical flyback control device (for example, LM5023, ICOMP=132μA) or a specific application method for an approximate PWM device.

  • 0 in reply to Akira Kurata
    Intellectual 280 points

    SOM8110_Additional questions　　　　　　　　

    Thank you very much for your answer regarding the previous question.

    I understood from the answer that the lower limit of practical current for the photocoupler emulator_ISOM8110 is 500μA.
    Based on your response, the operating state of this emulator, which is used as an alternative coupler for an existing switching power supply that is currently being evaluated at our company, is close to this lower limit current. From the measurement results of the G-P characteristics, it is predicted that stable control cannot be achieved near the FT frequency in the high range of the control band.
    However, we have not received a response from your company regarding all of our questions.
    Once again, I would like to ask you to answer the following questions.
    <Re-question>
    (1) We recognize that the control current of the feedback signal input terminal of most switching power supply controllers currently on the market, including your company's PWM and LLC control ICs, is 500μ or less in the actual control state.
    Under these conditions, wouldn't it be problematic to replace the current ISOM8110 with an existing photocoupler replacement, as there is basically no operating margin in the control current range?
    Also, even if alternative operation is possible, stable performance may not be maintained due to the characteristics of the alternative circuit application for the reasons mentioned above.
    (2) If question (1) above is answered in the affirmative, this device is clearly intended for use as a switching power supply. Therefore, if it is an appropriate application, please introduce an application that uses this photocoupler emulator and allows the control current to be set in the stable recommended range (1 mA, etc.) far exceeding 500 μA.
    Please give us an example of a typical flyback control device (for example, LM5023, ICOMP=132μA) or a specific application method for an approximate PWM device.

  • 0 in reply to Akira Kurata
    TI__Mastermind 29595 points

    Hello Kurata-san,

    Thank you for your patience and kindly restating the question. I have hopefully addressed each question below. 

    Akira Kurata said:
    (1) We recognize that the control current of the feedback signal input terminal of most switching power supply controllers currently on the market, including your company's PWM and LLC control ICs, is 500μ or less in the actual control state.
    Under these conditions, wouldn't it be problematic to replace the current ISOM8110 with an existing photocoupler replacement, as there is basically no operating margin in the control current range?

    Yes, the 500uA minimum forward current can be problematic (reasons explained in the initial reply). There is some margin on this limit, however applications like LM5023, the internal bias will limit the ICOMP below what the ISOM8110 can support.  

    Akira Kurata said:
    Also, even if alternative operation is possible, stable performance may not be maintained due to the characteristics of the alternative circuit application for the reasons mentioned above.

    This is not always the case. For example, LM51551 will work with ISOM8110. Since the ISOM8110 has a minimum current of 500uA, the controller's COMP pin needs to be biased externally (or have a large enough internal bias resistor) to work within the forward current range of ISOM8110. 

    Akira Kurata said:
    (2) If question (1) above is answered in the affirmative, this device is clearly intended for use as a switching power supply. Therefore, if it is an appropriate application, please introduce an application that uses this photocoupler emulator and allows the control current to be set in the stable recommended range (1 mA, etc.) far exceeding 500 μA.
    Please give us an example of a typical flyback control device (for example, LM5023, ICOMP=132μA) or a specific application method for an approximate PWM device.

    LM51551 would be a good example of ISOM8110 working to replace an opto-coupler. The drop-in-replace capability can be tested using the PMP22764 reference design. The minimum isolated compensation current will be a little greater than 1mA because of the way the COMP pin is biased, therefore ISOM8110 will work for this application. 

    The ISOM8110 is a new device, and your feedback here is highly appreciated. If you can provide any schematics or the requirements of your application that would be appreciated. 

    Best,
    Andrew