VSX60 & VSX75 APPLICATION NOTES

Transcription

1 Application Guide DCAN-40 Rev F VSX60 & VSX75 APPLICATION NOTES The VSX60 Series and VSX75 Series are 60 Watt and 75 Watt, respectively, dual output converters with 18-36V (VSX60 only) and 36-75V input models with 3.3VDC and 5VDC outputs. The VSX60 and VSX75 converters are packaged in the industry quarter-pak size of 1.5 x 2.3 x 0.5 and have a typical efficiency of 90%. The following application information is designed to supplement the product data sheet. Additional Information On The Control Features Remote On/Off Control The VSX60 Series and VSX75 Series are equipped with a primary on/off control pin for increased system flexibility. The input is TTL open-collector and/or CMOS open-drain compatible. On the standard model, the primary on/off pin uses negative logic, which turns the converter ON when a logic low signal is applied (see common specifications for threshold voltage levels). Figure 1 illustrates the typical external connections to enable this function. If no connection is made to the primary on/off terminal, the converter will not operate. Figure 1: Primary On/Off Connections In addition there is -1 option for the VSX60 Series and VSX75 Series (i.e. VSX60MD35-1) which allows the converters to operate on positive logic. The recommended control circuits would be the same as Figure 1 except the input logic would be reversed (i.e. input logic low will turn off the converter and input logic high will turn on the converter). If no connection is made to the primary on/off terminal of the converter with the -1 option, the converter will operate normally. Output Voltage Trim The output voltage trim feature allows the user to accurately adjust the converter's output voltage set point to a specified level (see Table 1). This is achieved by connecting a resistor from the TRIM terminal to either the 3V3 (+3.3 Volt Output) or O/P RTN (Output Return) pins. If an increased output voltage is desired, the resistor should be connected between the TRIM and O/P RTN pins. If a decreased output voltage is desired, the external resistor should be connected between the TRIM and 3V3 pins. Figure 2 illustrates the required connections to implement this feature. Connecting the TRIM pin directly to the 3V3 pin will trim the converter down 10% and connecting the TRIM pin directly to the O/P RTN pin will trim the converter up 10%. Please Note: When using the Page 1

2 output voltage Trim feature, both outputs will be adjusted in the same direction and by the same percentage. TABLE 1 - RESISTOR (R UP ) VALUES (KOHMS) FOR DESIRED INCREASE IN OUTPUT VOLTAGE (%) MODEL VSX60xS RESISTOR (R DOWN ) VALUES (KOHMS) FOR DESIRED DECREASE IN OUTPUT VOLTAGE (%) MODEL VSX60xS Design Considerations Current Limiting Each converter is equipped with current-limiting circuitry designed to provide continuous protection against fault or short circuit conditions. The current limit point is typically 5%-25% above the rated output current. When the short circuit condition is removed, the output voltage will return to its pre-short circuit value without recycling the input. Safety The VSX60 Series and VSX75 Series are recognized to be in conformance with UL/CUL1950 and EN The output of the VSX60 and VSX75 are considered to be a level 3 output when the supply to the converter meets SELV requirements (VIN < 60VDC). Fusing Requirements The VSX60 Series and VSX75 Series converters are not internally fused. In order to maintain maximum safety and overall system protection, an input line fuse should always be included. A normal blow D.C. fuse with a maximum rating of 6A is recommended. Input Source Impedance The converter should be connected to a low AC impedance input source to ensure system stability. It is recommended that a small electrolytic capacitor with a low ESR (eg. 33µF, 0.5Ω) for 48V input models (M), 330µF, 0.14Ω for 24V input models (L)) should be placed across the input terminals as close to the module as possible to maintain unit stability. Page 2

3 Test Circuit Configurations Figure 3: Measurement of Input Reflected Ripple Figure 4: Measurement of Input Reflected Ripple with External Reduction Circuit Figure 5: Measurement of Peak to Peak Output Noise Circuit Configuration Designed to Minimize Conducted EMI Minimizing conducted EMI from a switching DC to DC converter can sometimes be a challenge. The VSX60 and VSX75 EMI filter circuit shown in Figure 6 and the circuit layout in Figure 7 was designed to simplify this challenge. This circuit can be used in most applications with little or no modification to meet most FCC requirements. Figure 6: VSX EMI Filter Circuit Page 3

4 Figure 7: Circuit Layout For VSX EMI Filter The following components are recommended for the Circuit in Figures 6 and 7 F1 6 Amp Normal Blow Fuse C µf Metallized Polyester Film Capacitors C2, C pf Ceramic Type 1500V Capacitors C5, C6 4.7 nf 1500 Volt Ceramic Capacitors C7, C8, C9, C µf 200 Volt Ceramic Type Capacitors C11, C µf 200 Volt Ceramic Type Capacitors C4 33 µf, 100 Volt Low ESR, Electrolytic Capacitor L1 Common-mode Inductor, Pulse P0351 or equivalent R1, R2 5 Ohm surface mount resistors (application dependent) Page 4

5 Figure 8 are actual graphs from a spectrum analyzer showing the conducted EMI from a fully loaded VSX60MS35 when used in conjunction with the recommended board layout and suggested components. The limit lines in the graphs are CISPR Class B, Quasi Peak emission limits. Figure 8: VSX60 Conducted EMI Using Recommended Board Layout and Components Thermal Considerations The VSX60 Series and VSX75 Series of products are designed to operate with a baseplate temperature between -40 degrees C. and +100 degrees C. If the baseplate temperature should rise above +100 degrees due to internal heat generation, an Over Temperature Protection (OTP) circuit will turn the VSX converter off to prevent any damage. The unit will turn itself back on when the baseplate temperature drops below 100 degrees C. The actual OTP shutdown temperature is between 110 and 125 degrees C. case. To avoid Page 5

6 unexpected shutdown, it is important to always keep the baseplate temperature below 100 degrees C. This can be done by first calculating the case temperature under the worst case conditions and determine if additional airflow, a heatsink or both maybe required. Thermal Equations In order to calculate the case temperature in a particular application, the following thermal equations will prove useful: 1) Efficiency = η = P OUT / P IN (P= Power) 2) Dissipated Power (Power Lost) = P D = P OUT {(1-η)/η} 3) Temperature Rise = P D x θ BA = P OUT x θ BA {(1-η)/η} (θ BA =Thermal Resistance from Base to Ambient) 4) Maximum Output Power = {T BASE (max) - T A } /[θ BA {(1-η)/η} (T BASE (max) = Maximum BaseTemperature = 100 degrees C) (T A = Ambient Temperature) 5) Minimum Thermal Impedance = {T base (max) - T A }/[P OUT {(1-η)/η} Normalized Efficiency Charts The following charts can be used to more precisely predict the expected efficiency of the VSX60 and VSX75: NORMALIZED EFFICIENCY VS TOTAL OUTPUT LOAD Normalized Efficiency Total Load (% Rated Power) Page 6

7 NORMALIZED EFFICIENCY VS LOAD DISTRIBUTION %/0% 75%/25% 50%/50% 25%/75% 0%/100% Normalized Efficiency LOAD% (3.3V / 5V) NORMALIZED EFFICIENCY VS INPUT VOLTAGE Normalized Efficiency Input Voltage (Volts) Page 7

8 NORMALIZED EFFICIENCY VS BASEPLATE TEMPERATURE Normalized Efficiency Degrees Celsius Thermal Impedance VSX THERMAL IMPEDANCE WITHOUT A HEATSINK θ BA (Degree C / Watt) Air Flow LFM Without Mechanical Tie With Mechanical Tie Down** Free Air **Mechanically tied down through (4) PEMS with steel screws to a 2 layer FR4. FR4 Top ground plane contains sq. in. of 2 oz copper and FR4 ground plane contains 16 sq. in of 2 oz. copper. The steel screws shall be applied from the baseplate side of the converter and a nut and washer shall be used to make the electrical connection to the FR4 ground plane. Applying the screws from the FR4 side of the converter will void the converter s warranty. The following example will illustrate how to calculate the baseplate temperature under specific operating conditions: Example: A customer has an application that requires a VSX60MD35 to drive a load of 53 watts in a cabinet at 50 degrees C. with 200 linear feet per minute of air flow. The input voltage will be typically 48 Volts but under some conditions may rise to 52 Volts. The load current on the VSX60MD35 will be distributed between the 5 Volt output and 3.3 Volt output as follows: 5.0 Volt load current = 4 Amps 3.3 Volt load current = 10 Amps In order to determine the baseplate temperature of the VSX60MD35, the amount of heat generated must be determined. Heat generated by converter caused by the inefficiency of the converter is Page 8

9 called Dissipated Power. The efficiency of the VSX60MD35 with a 53 Watt load, under the stated conditions, can be calculated as follows: Efficiency =η=η pds x η vin x η tld x η ldd = (.89) x (.997) x (1.00) x (.996) = 88.38% Where: η pds = Minimum Efficiency of VSX60MD35 from data sheet = 89% η vin = Efficiency adjustment factor based on the graph of Normalized Efficiency Vs. Input Voltage η tld = Efficiency adjustment factor based on the graph of Normalized η ldd = Efficiency adjustment factor based on the graph of Normalized Efficiency Vs. Total Output Load Efficiency Vs. Load Distribution Because efficiency is defined as Power-Out (P OUT ) divided by Power-In (P IN ) times 100%, the Dissipated Power (P D ) can be calculated as follows: Efficiency/100% = η = P OUT /P IN = (V OUT x I OUT )/(V IN x I IN ) Dissipated Power = P D = P OUT {(1-η)/η} = 53 x {( )/0.8838} Dissipated Power = Watts Next, the temperature rise of the baseplate can be determined by multiplying the thermal resistance from base plate to ambient (θba) which can be found in the Thermal Impedance chart on page 8 times the Dissipated Power. By knowing the temperature rise of the baseplate, the worst case temperature of the baseplate can be determined by adding the ambient temperature T A which is shown as follows: Temperature Rise= T R =P D x θ BA =P OUT x θ BA {(1-η)/η} = x 7.49 = degrees C. Temperature of the baseplate =T BP =T R xt A = = degrees C. With a baseplate temperature of degrees C., the converter will operate outside of its specified temperature range. There are several ways to solve this problem: drop the thermal resistance from baseplate to ambient; reduce the output power; or lower the maximum ambient temperature. The thermal resistance can be reduced by blowing additional air over the converter; adding a heat sink; or both. The following equations can be used to calculate the maximum output power that can be delivered or the maximum thermal resistance the converter can tolerate. The maximumthermalresistancecanthenbeusedinconjunctionwiththethermalimpedancecharton page 14 to determine the correct combination of air flow and heat sink. Note: the combination in the chart should have a lower thermal resistance than the maximum calculated. In the this example, if nothing else is changed, the maximum output power would have to be reduced by using Equation 4 on page 6 as follows: Maximum Output Power = {Tbaseplate (max)-ta} /[θ BA {(1-η)/η} Maximum Output Power = (95-50)/[7.49 x {( /0.8838)}] Note: 95 degrees C. was used instead of 100 degrees C. to add 5 degrees C. of margin. Maximum Output Power = 45.6 Watts Page 9

10 In this example, if the thermal Impedance can be changed, Equation 5 on page 6 can be used to calculate the required Maximum Thermal Impedance as follows: Maximum Thermal Impedance = θ BA ={Tbaseplate (max)-ta}/[p OUT {(1-η)/η} Maximum Thermal Impedance = θ BA = (95-50)/ [53 x {( /0.8838)}] Maximum Thermal Impedance = θ BA = Degrees C./ Watt As can be seen by looking at the thermal impedance charts on the page 8 and page 14, this example would require 400 Linear Feet per Minute (LFM) of airflow with no heat sink or a VSX60 or VSX75 with HSK-VSX heat sink and 200 Linear Feet per Minute (LFM) of airflow. Power Derating Information The following graphs show (as an alternative) the power derating of the VSX60 and VSX75 as a function of ambient temperature and air flow: Page 10

11 POWER DERATING FOR U VERSIONS (Unencapsulated) Output Load (I1 3.3V Output) v/s Ambient Temperature (TA) (VSX60MD35-U w/o Mechanical Tiedown) I1 3.3V Output (A) 10 8 No Air Flow 100 LFM 200 LFM 300 LFM 400 LFM TA (Deg C) 12 Output Load (I2 5.0V Output) v/s Ambient Temperature (TA) (VSX60MD35-U w/o Mechanical Tiedown) 10 I 5.0V Output (A) 8 No Air Flow 100 LFM 200 LFM 300 LFM 400 LFM TA (Deg C) Page 11

12 Output Load (I1+I2) v/s Ambient Temperature (TA) (VSX60MD35-U w/o Mechanial Tiedown) I1+I2 (% of full load on each outputs) TA (Deg C) No Air Flow 100 LFM 200 LFM 300 LFM 400 LFM Output Load (I1 3.3V Output) v/s Ambient Temperature (TA) (VSX75MD35-U w/o Mechanical Tiedown) I1 3.3V Output (A) No Air Flow 100 LFM 200 LFM 300 LFM 400 LFM TA (Deg C) Page 12

13 Output Load (I2 5.0V Output) v/s Ambient Temperature (TA) (VSX75MD35-U w/o Mechanical Tie-down) 14 I 5.0V Output (A) No Air Flow 100 LFM 200 LFM 300 LFM 400 LFM TA (Deg C) Output Load (I1+I2) v/s Ambient Temperature (TA) (VSX75MD35-U w/o Mechanical Tiedown) 50.0 I1+I2 (% of full load on each outputs) No Air Flow Series1 100 LFM 200 LFM 300 LFM 400 LFM TA (Deg C) Page 13

14 Recommended Accessories Following is a drawing of heat sink HSVSX from C&D Technologies which has been designed to be used with the VSX60 and VSX75 families of products. VSX Thermal Impedance* θ ΒΑ (Deg Air Flow LFM C/Watt) With Heat sink HSVSX Free Air *Thermal impedance includes the thermal impedance of 0.1 degree C. per Watt of a thermally conductive interface material between VSX60 and VSX75 baseplate and heat sink. KIT*/PART # HEAT SINK INCLUDED IN KIT OVERALL PACKAGE HEIGHT** HSK-VSX HSVSX 0.75" * The complete kits also include four mounting screws and a precut thermal interface material to provide improved thermal conductivity from the converter to the heat sink **The overall package height includes a 0.01" height for the interface pad. C&D Technologies tries to maintain a reasonable stock of the above heatsink kit. Please either contact the factory or one of our authorized distributors for price and availability Page 14

15 For answers to questions on information contained in this application note, please contact C & D Technologies, DC to DC Applications Group, by phone at (520) , by Fax at (520) , at swood@cdtechno.com. or write us at the address shown below. HISTORY OF CHANGES Rev A Fusing on page 2 changed from 10A to 6A. EMI filter changed on pages 3-5 Rev B EMI filter changed on pages 3-5 and thermal example added on pages Rev C Various corrections in grammar Rev D Added on page 8, the recommended method of mechanical tie down to FR4 ground plane. Changed grounding recommendations on page 2 under Safety Updated Applications group Address on page 12. Rev E Added VSX75 Series throughout App Note. Changed Safety Agency Nomenclature on page 2. Changed Normalized Efficiency Graph on page 6. Changed both Power Derating Graphs on page 10. New address and contact information on Page 10. Rev F Added 6 new Derating Graphs for Unencapsulated VSX60 and VSX 75 on Pages Page 15