Design guide IDP2303(A)
General features and system design considerations
Application Note
38
Revision 2.0
2017-05-03
5.2
LLC High-side VCC (HSVCC) cap
To optimize LLC switching behaviour during start-up and burst mode operation, HSVCC cap is targeted to
reach its turn-on threshold within the first low-side gate pulse. Thus, its capacitance can be calculated as:
𝐶
𝐻𝑆𝑉𝐶𝐶
< −
𝑡
ln (1 −
𝑉
𝐻𝑆𝑉𝐶𝐶_𝑜𝑛
𝑉
𝑐𝑐
− 𝑉
𝐷
) ∗ 𝑅
𝐻𝑆𝑉𝐶𝐶
Considering LLC softstart frequency as 270kHz with 500ns dead time, and HSVCC on threshold V
HSVCC_on
as
9.7V, V
cc
as 15V, V
D
as 0.7V and R
HSVCC
as 10
Ω
, thus the HSVCC cap C
HSVCC
can be calculated as :
𝐶
𝐻𝑆𝑉𝐶𝐶
< 119𝑛𝐹
So in the design, a 100nF cap is recommended.
5.3
Touch current
There is an industrial standard for touch voltage/current that has to be met by the power supply. For
example, IEC60065 specifies the limit for Class II equipment as 0.35 V/0.70 mA peak. In tropical climates the
values given have to be halved so the limit becomes 0.175 V/0.35 mA peak.
Usually the challenge comes from standby (burst mode) operation: when the AC voltage is high (e.g. near
the AC peak), the voltage across the Y-capacitor (between primary ground & secondary ground) is charged to
a high value during burst-on. During burst-off, the voltage across the Y-capacitor is maintained and the
energy remains as there is no discharge path. When the next burst-on or AC detection via the HV pin occurs,
the AC voltage may be at a much lower value, then the Y-capacitor will be discharged through all possible
discharge paths and thus causes spikes in the touch current. This spike current may exceed the 0.35 mA limit
as defined by IEC60065.
During normal operation, the voltage across the Y-capacitor will follow the AC voltage through the cycle, and
no touch current spike will occur.
The following are some design considerations in order to meet the touch current:
Use a relatively small Y-capacitor between primary ground and secondary ground, so that the energy
stored in is relatively small. The value is selected based on the tradeoff between the EMI/lightning surge
requirement (where a bigger capacitor is preferred) and the touch current requirement.
Add discharging resistor in parallel with the Y-capacitor. The value is selected based on the tradeoff
between the standby power losses and the discharge speed, thus a 1~3 MΩ resistor value might be
appropriate.
Add a resistor (1~3 mΩ) and/or high-voltage capacitors (around 1 nF) in parallel with the bridge diodes to
provide an alternative discharge path.
Add a resistor (1~3 mΩ) from HV to the primary-side GND to provide an alternative discharge path.
5.4
Black box
The black box function is responsible for sending diagnostic data through the UART (MCOM) pin to a host
MCU or for probing directly at the pin.
The UART (MCOM) pin will be configured as a digital output pin after protection is triggered, and will output
the relevant diagnostic data digitally.
