Datasheet ADP1031 (Analog Devices) - 34

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ADP1031 Data Sheet INSULATION LIFETIME
2 2 = R V + MS VACRMS DC V (1) All insulation structures eventual y break down when subjected to voltage stress over a sufficiently long period. The rate of insulation or degradation is dependent on the characteristics of the voltage 2 2 V = V −V (2) waveform applied across the insulation as wel as on the materials AC RMS RMS DC and material interfaces. where: The two types of insulation degradation of primary interest are VRMS is the total rms working voltage. breakdown along surfaces exposed to the air and insulation wear VAC RMS is the time varying portion of the working voltage. out. Surface breakdown is the phenomenon of surface tracking VDC is the dc offset of the working voltage. and the primary determinant of surface creepage requirements
Calculation and Use of Parameters Example
in system level standards. Insulation wear out is the phenomenon The following example frequently arises in power conversion where charge injection or displacement currents inside the applications. Assume that the line voltage on one side of the insulation material cause long-term insulation degradation. isolation is 240 V ac rms and a 400 V dc bus voltage is present
Surface Tracking
on the other side of the isolation barrier. The isolator material is Surface tracking is addressed in electrical safety standards by polyimide. To establish the critical voltages in determining the setting a minimum surface creepage based on the working voltage, creepage, clearance, and lifetime of a device, see Figure 75 and the environmental conditions, and the properties of the insulation the following equations. material. Safety agencies perform characterization testing on the surface insulation of components that al ows the components to be categorized in different material groups. Lower material group
E
ratings are more resistant to surface tracking. Therefore, lower
AG VAC RMS
material group ratings provide adequate lifetime with smaller
LT O V
creepage. The minimum creepage for a given working voltage and material group is determined in each system level standard and is
ATION VPEAK VRMS VDC
based on the total rms voltage across the isolation, pollution
OL IS
degree, and material group. The material group and creepage for the ADP1031 isolators are shown in Table 4. 375
Insulation Wear Out TIME
16434- The lifetime of insulation is determined by thickness, material Figure 75. Critical Voltage Example properties, and the voltage stress applied. It is important to The working voltage across the barrier from Equation 1 is verify that the product lifetime is adequate at the application working voltage. The working voltage supported by an isolator for 2 2 = R V + MS VACRMS DC V wear out may not be the same as the working voltage supported for tracking. The working voltage applicable to tracking is 2 2 V = RMS 240 + 400 specified in most standards. V Testing and modeling have shown that the primary driver of RMS = 466 V long-term degradation is displacement current in the polyimide This VRMS value is the working voltage and is used together with insulation. This displacement current causes incremental damage the material group and pol ution degree when looking up the to the insulation. The stress on the insulation can be broken down creepage required by a system standard. into broad categories: dc stress and ac component time varying To determine if the lifetime is adequate, obtain the time varying voltage stress. DC stress causes very little insulation wear out portion of the working voltage. To obtain the ac rms voltage, because there is no displacement current. AC component time use Equation 2. varying voltage stress causes insulation wear out. 2 2 V = V −V The ratings in certification documents are usually based on AC RMS RMS DC 60 Hz sinusoidal stress because this reflects isolation from line 2 2 V = AC RMS 466 − 400 voltage. However, many practical applications have combinations of 60 Hz ac and dc across the barrier as shown in Equation 1. VAC RMS = 240 V rms Because only the ac portion of the stress causes wear out, the In this case, the ac rms voltage is simply the line voltage of equation can be rearranged to solve for the ac rms voltage, as 240 V rms. This calculation is more relevant when the waveform is shown in Equation 2. For insulation wear out with the polyimide not sinusoidal. The value is compared to the limits for working materials, the ac rms voltage determines the product lifetime. voltage in Table 8 for the expected lifetime, which is less than a Rev. A | Page 34 of 38 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION TYPICAL APPLICATION CIRCUIT COMPANION PRODUCTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS REGULATORY INFORMATION ELECTROMAGNECTIC COMPATIBILITY INSULATION AND SAFETY RELATED SPECIFICATIONS DIN V VDE 0884-10 (VDE V 0884-10) INSULATION CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION FLYBACK REGULATOR Flyback Regulator Operation Power Saving Mode (PSM) Flyback Undervoltage Lockout (UVLO) Flyback Regulator Precision Enable Control Flyback Regulator Soft Start Flyback Slew Rate Control Flyback Regulator Overcurrent Protection Flyback Regulator Overvoltage Protection BUCK REGULATOR Buck Regulator Operation Buck Regulator UVLO Buck Regulator Soft Start Buck Regulator Current-Limit Protection Buck Regulator OVP Buck Regulator Active Pull-Down Resistor INVERTING REGULATOR Inverting Regulator Operation Inverting Regulator UVLO Inverting Regulator Soft Start Inverting Regulator Current-Limit Protection Inverting Regulator OVP Inverting Regulator Active Pull-Down Resistor POWER GOOD POWER-UP SEQUENCE OSCILLATOR AND SYNCHRONIZATION THERMAL SHUTDOWN DATA ISOLATION High Speed SPI Channels GPIO Data Channels APPLICATIONS INFORMATION COMPONENT SELECTION Feedback Resistors Capacitor Selection FLYBACK REGULATOR COMPONENTS SELECTION Input Capacitor Output Capacitor Ripple Current vs. Capacitor Value Schottky Diode Transformer Turn Ratio Primary Inductance Flyback Transformer Saturation Current Series Winding Resistance Leakage Inductance and Clamping Circuits Clamping Resistor Clamping Capacitor Clamping Diode Diode Zener Diode Clamp Ripple Current (IAC) vs. Inductance Maximum Output Current Calculation BUCK REGULATOR COMPONENTS SELECTION Inductor Output Capacitor INVERTING REGULATOR COMPONENT SELECTION Inductor Output Capacitor Inverting Regulator Stability INSULATION LIFETIME Surface Tracking Insulation Wear Out Calculation and Use of Parameters Example THERMAL ANALYSIS TYPICAL APPLICATION CIRCUIT PCB LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE