AC-DC|Design
Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
2019.04.11
Points of this article
・Components necessary for the different detection pins should be set according to the data sheet and the design manual.
・If noise enters the detection pins, erroneous operation and other problems may result. The addition of a capacitor or RC filter should be considered.
This article discusses components necessary for various detection pins used for a power supply IC to perform step-down and regulation control.
Current Sense Resistor:R19
R19 converts the current flowing to the primary side due to the switching transistor Q1 into a voltage. This voltage is monitored by the CS pin, and an output overload protection point is set. The value of R19 is set such that when the primary-side maximum current Ippk (0.66 A) flows, the CS pin overcurrent detection voltage is Vcs = 1 V.
→1.5Ω
Moreover, the loss P_R19 of the resistor R19 is
Considering pulse resistance, P_R19 is assumed to be 1 W or higher.
Pulse resistance may change with the structure of the resistor and the like even for the same power rating. The manufacturer of the resistor used should be consulted.
Resistor and Capacitor for CS Pin Noise Protection: R22, C13
When noise is not completely eliminated by the blanking function, this RC filter is added. Even when the filter is not necessary, it is recommended that R22 (value approx. 1 kΩ) be inserted as a surge countermeasure. The value of C13 is about 47 pF.
ZT Pin Voltage Setting Resistor: R21
R21 sets the bottom detection voltage at the ZT pin. The bottom detection voltage at the ZT pin is Vzt1 = 100 mV typ. (ZT pin voltage falling) and Vzt2 = 200 mV typ. (ZT pin voltage rising). From ZTOVP(min) = 3.30 V, as a rule Vzt is set to approximately 1 to 3 V. R20 was set to 100 kΩ as per ” Switch Setting Resistors for Overload Protection Points“. The number of windings is 8 turns for both Nd and Ns, as calculated in “Transformer T1 Design – 2“.
Upon setting,
→12kΩ
ZT_Pin Capacitor:C11
The capacitor C11 is used for ZT pin regulation and to adjust the bottom detection timing. It should be set while checking the ZT pin waveform and bottom detection timing. In this circuit example, it is set to 47 pF.
FB Pin Capacitor:C12
C12 is a capacitor for FB pin regulation. A value of approximately 1000 pF to 0.01 μF is recommended. In this circuit example, it is set to 2200 pF.
List of articles related to the「Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins」
- Introduction
- Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- Design Example Circuit
- Transformer T1 Design – 1
- Transformer T1 Design – 2
- Selecting Critical Components: MOSFET Q1
- Selecting Critical Components: Input Capacitor and Balancing Resistor
- Selecting Critical Components: Switch Setting Resistors for Overload Protection Points
- Selecting Critical Components: VCC-Related Components of Power Supply ICs
- Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins
- Selecting Critical Components: Components Related to Snubber Circuits
- Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit
- Selecting Critical Components: Output Rectifying Diode
- Selecting Critical Components: Output Capacitors, Output Setting and Control Components
- Selecting Critical Components: Components for Dealing with EMI and Output Noise
- PCB Layout Example
- Example Circuit and Component List
- Evaluation Results: Efficiency and Switching Waveform
- Summary
Download Technical Documents
Basic of AC-DC Conversion
Basic studies to understand AC-DC converters and to go designing.
AC-DC
- Basic
-
Design
-
Overview of Design Method of PWM AC-DC Flyback Converters
- Want are Isolated Flyhback Convertors?
- Isolated Flyback Converter Basics: What is Switching AC-DC Conversion?
- Isolated Flyback Converter Basics: What are Characteristics of Flyback Converter?
- Isolated Flyback Converter Basics: Flyback Converter Operation and Snubber
- Isolated Flyback Converter Basics: What are Discontinuous Mode and Continuous Mode?
- Design Procedure
- Determining Power Supply Specifications
- Choosing an IC for Design
- Designing Isolated Flyback Converter Circuits
- Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating numerical values)
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 1
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 1
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? CIN and Snubber
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? Output Rectifier and Cout
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? VCC of IC
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components – IC Settings Etc.
- Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise
- Example Board Layout
- Summary
-
Overview of Design Examples of AC-DC Non-isolated Buck Converters
- What are Buck Converters? – Basic Operation and Discontinuous Mode vs. Continuous Mode
- Selection of Power Supply ICs and Design Examples
- Selecting Critical Components: Input Capacitor C1 and VCC Capacitor C2
- Selecting Critical Components: Inductor L1
- Selecting Critical Components: Current Sense Resistor R1
- Selecting Critical Components: Output Capacitor C5
- Selecting Critical Components: Output Rectifying Diode D4
- EMI Countermeasures
- Board Layout and Summary
-
Introduction
- Design Procedure
- IC Used in Design
- Power Supply Specifications and Replacement Circuit
- Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET
- Synchronous Rectification Circuit Section: Power Supply IC Selection
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-C1, R3 at MAX_TON Pin, and VCC Pin
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-D1, R1, R2 at DRAIN Pin
- Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components
- Troubleshooting ①: Case When Secondary-Side MOSFET Suddenly Turns OFF
- Troubleshooting ②: Case When Secondary-Side MOSFET Turns On Due to Resonance Under Light Loading
- Troubleshooting ③: Case When, Due to Surge, VDS2 Rises to Above Secondary-Side MOSFET VDS Voltage
- Comparison of Efficiency of Diode Rectification and Synchronous Rectification
- Points to Note Relating to PCB Layout
- Summary
-
Introduction
- Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- Design Example Circuit
- Transformer T1 Design – 1
- Transformer T1 Design – 2
- Selecting Critical Components: MOSFET Q1
- Selecting Critical Components: Input Capacitor and Balancing Resistor
- Selecting Critical Components: Switch Setting Resistors for Overload Protection Points
- Selecting Critical Components: VCC-Related Components of Power Supply ICs
- Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins
- Selecting Critical Components: Components Related to Snubber Circuits
- Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit
- Selecting Critical Components: Output Rectifying Diode
- Selecting Critical Components: Output Capacitors, Output Setting and Control Components
- Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
- Selecting Critical Components: Components for Dealing with EMI and Output Noise
- PCB Layout Example
- Example Circuit and Component List
- Evaluation Results: Efficiency and Switching Waveform
- Summary
-
Overview of Design Method of PWM AC-DC Flyback Converters
- Evaluation
- Product Information
- FAQ